Open the SETTINGS menu and it should open into a tab called GAMEPLAY.
From here you need to scroll all the way down.
At the bottom there should be a setting called "Enable Developer Console:".
If this option is set to OFF, then you need to switch it to ON.
After you have done that, press the tilde key (~) on your keyboard.
The console should open, no matter if you are in game or on the main menu.

Use 1 or 0 after commands that turn things on/off, for example, "cheats 1" will turn cheats on, and "cheats 0" will turn cheats off.

cheats - Turns cheats on/off
antialiasing - Changes the anti-aliasing sample amount
bloom - Turns bloom on/off
clear - Clears the console text
crosshair - Turns the crosshair on/off
difficulty - Changes the difficulty level

• 0 = Crescent Moon (Easy)
  
• 1 = Half Moon (Normal)
  
• 2 = Full Moon (Hard)
  
• 3 = Blood Moon (Expert)

fog - Turns the fog on/off
fov - Sets the field of view value
framerate - Sets the max fps cap
hud - Turns the hud on/off
lightflares - Turns light flares on/off
tooltips - Turns tooltips on/off
version - Says the current version of the game
shaderwarmup - Warm up the shaders loaded into memory
texturefiltering - Sets the texture filtering setting
get - Get values and state by input
place - Probably spawns entity markers, like patrol patterns or something

help - Shows a bunch of commands you can use
list - Lists values from entities in the game
ghost - Noclip and notarget combined into one command
give - Puts an item of your choice into your inventory (unless its full)
spawn - Spawns various things of your choice into the world
god - Makes you invincible
infinite - "infinite ammo true" should give the weapon you're holding infinite ammo
level - Loads you into a level of your choice
life - Sets your health to an amount of your choice (0 kills you, 100 fully heals you)
savegame - Saves your game
loadgame - Loads your game
noclip - Grants you the power of flight
notarget - Makes enemies ignore you
quit - I'm pretty sure it's obvious what this one does
restart - Restarts the level you're on
thief - Makes you very hard to see (indicated by the light gem ring turning black) and able to open locked doors without having the key
fullbright - Doesn't seem to do anything
timescale - Alters the speed of the game (1 is normal speed, above 1 is faster speed)
kill - Kills the thing your crosshair is on (works on destructible objects too)
gib - Kills the thing your crosshair is on (works on destructible objects too)
set inventory size (x,y) - Changes the size of your inventory, (6,4) being the default
research - Research an item
beastform (beast) (time) - Use the serum effects for a specified amount of time

These are meant to be used with either the give or spawn commands.
Replacing weapon with item works too, like item_canesword.

weapon - Spawns all weapons
weapon_bonesaw - Spawns the TF2 Medic's melee weapon
weapon_canesword - Spawns the cane sword
weapon_revolver - Spawns a revolver
weapon_shotgun - Spawns a shotgun
weapon_undertaker - Spawns the undertaker. weapon_flaregun works too
weapon_harpoongun - Spawns the harpoon gun
weapon_legtrap - Spawns a beartrap
weapon_handlantern - Spawns a lantern
weapon_lightgemring - Spawns a light gem ring
weapon_flashbomb - Spawns a flashbomb
weapon_chokebomb - Spawns a chokebomb
weapon_jackolantern - Spawns a carved turnip-o-lantern (Only spawnable during Halloween)
weapon_snowball - Spawns a snowball (Only spawnable during Christmas)

These are meant to be used with the give or spawn commands.

item_revolverround - Spawns a revolver bullet
item_revolverroundbox - Spawns a box of bullets
item_revolverroundspent - Spawns a used revolver casing
item_shotgunshell - Spawns a shotgun shell
item_shotgunshellbox - Spawns a box of shells
item_shotgunshellspent - Spawns a used shotgun shell
item_flaregun_baton - Spawns a baton bolt. item_undertaker_slug is the same
item_flaregun_flare - Spawns a flare bolt
item_flaregun_incendiary - Spawns an incendiary bolt. item_undertaker_incendiary is the same
item_flaregun_choke - Spawns a choke bolt. item_undertaker_choke is the same
item_undertaker_shock - Spawns a shock bolt No longer exists
item_harpoon_rope - Spawns a harpoon
item_harpoon_rope_bundle - Spawns a bundle of harpoons
item_cannonball - Spawns a cannon ball

These are meant to be used with the give or spawn commands.

item - Tries to give you every item
item_apple - Spawns an apple
item_potato_# - Spawns a potato. Replace # with 1, 2, 3, or 4 to get different types of potato
item_carrot_# - Spawns a carrot. Replace # with 1, 2, or 3 to get different types of carrot
item_carrot_1_chopped - Spawns a chopped carrot
item_onion_1 - Spawns an onion
item_salsify_1 - Spawns a salsify root vegetable
item_driedpepper_# - Spawns a dried pepper. Replace # with 1 or 2
item_mushroom_1 - Spawns an edible mushroom
item_corncob_1 - Spawns a corn cob
item_toast_1 - Spawns toast. Use toast_2 to spawn a halved slice of toast
item_sandwich_brain_1 - Spawns a brain sandwich
item_breadloaf_1 - Spawns a loaf of bread
item_breadloaf_1_half - Spawns half of a loaf of bread
item_breadloaf_1_slice - Spawns the best thing
item_meat_brain_1 - Spawns a brain
item_meat_offal_1 - Spawns offal
item_meat_salami_slice_# - Spawns a slice of salami. Replace # with 1, 2, 3, or 4
item_cheese - Spawns a cheese wheel
item_cheese_wedge_1 - Spawns a wedge of cheese
item_bottle - Spawns a bottle. item_bottle_1 spawns the same thing
item_bottle_wine_1 - Spawns a wine bottle
item_brooch_1 - Spawns a brooch
item_button_presser - Spawns a button
item_can_beans - Spawns a can of beans
item_can_beans_empty - Spawns an empty can of beans
item_can_fish - Spawns a can of fish
item_can_fish_empty - Spawns an empty can of fish
item_tincan_# - Spawns a tin can. Replace # with 1 or 2
item_boot_1_# - Spawns a boot. Replace # with left or right
item_cheese - Spawns cheese
item_coin - Spawns a coin (Singular coin gives you 9999 coins, spawning multiple gives you a random amount of coins for each one you spawned) item_coin_pouch does the same thing
pickup_coin - Spawns a coin (pick up for 2 coins)
item_coin_ancient - Spawns an ancient coin
pickup_coin_pouch - Spawns a coin pouch (pick up for 25 coins)
item_gem_raw_large_1 - Spawns a large green gem
item_gem_raw_small_1 - Spawns a small gem
item_handlantern - Spawns a lantern
item_handlantern_broken - Spawns a broken lantern
item_lightgemring - Spawns the light gem ring
item_medhypo - Spawns a healing syringe
item_rifle_broken - Spawns a Huntsman's rifle
item_waxcylinder - Spawns a wax cylinder
item_powderflask - Spawns a powderflask horn
item_cannon_fuse - Spawns a fuse
item_gunpowder - Spawns gunpowder (it doesn't have physics)
item_valve - Spawns a valve. item_valve_wheel seems to do the same thing
item_horn_ulmar - Spawns the horn of Ulmar
item_bloodvial - Gives you a blood vial
item_gem_blood_small - Spawns a small blood gem
item_gem_blood_tiny - Spawns a tiny blood gem
item_gem_emerald_large - Spawns a large emerald
item_gem_white_large - Spawns a large white gem
item_gem_white_small - Spawns a small white gem
item_journal - Gives you a second journal
item_luggagecase - Gives you your inventory
item_fishery_power_room_key - Gives yourself a fishery power room key
item_pumpkin - It spawns something somewhere, might be invisible
item_oil - It spawns something somewhere, might be invisible
item_barrel_oil - Spawns a barrel of oil
item_monocle - Spawns a monocle
item_pocketwatch_1_gold - Spawns a gold pocketwatch
item_chalice_1_gold - Spawns a gold chalice
item_crank_handle - Spawns a crank handle
item_crank_handle_gold - Spawns a gold crank handle
item_key_medium_# - Spawns keys. Replace # with rust, black, brass, iron, or gold
item_brick_1 - Spawns a brick
item_seal_underport - Spawns the Underport seal
item_upgrade_luggage_8x6 - Spawns the inventory upgrade
item_ingot_gold - Spawns a pouch, probably a placeholder
item_serum_failed - Spawns a failed serum
item_serum_huntsman - Spawns a huntsman serum
item_serum_batbarber - Spawns a Bat Beast serum
item_serum_crowman - Spawns a Crow Crook serum
item_serum_crowking - Spawns a crow king serum
item_serum_fishdog - Spawns an aquatic doggy serum
item_serum_goatman - Spawns a Goat Man serum
item_research_jar_## - Spawns jars. Replace # with a number from 1 to 20
item_research_jar_1_eyeballs - Spawns a jar of eyeballs
item_research_jar_2_mouth - Spawns a jar containing a swollen Gulper mouth
item_research_jar_3_teeth - Spawns a jar of hardened enamel teeth
item_research_jar_4_tendrils - Spawns a jar of tasty tendrils!
item_research_jar_5_hand - Spawns a vat containing pruney hands
item_research_jar_6_leeches - Spawns a jar of engorged leeches in a brackish water
item_research_jar_7_tentacles - Spawns a jar of deep sea tentacles
item_research_jar_8_organs - Spawns a jar of organs (broken description)
item_research_jar_9_intestines - Spawns a jar of intestines (broken description)
item_research_jar_10_worms - Spawns a jar containing a cluster of slimy, squirming worms
item_research_jar_11_tongues - Spawns a jar of animal tongues
item_research_jar_12_fingers - Spawns a jar of severed human fingers
item_research_jar_13_doll - Spawns a jar containing a haunted doll
item_research_jar_14_eyeball - Spawns a jar containing... a brain? (broken description)
item_research_jar_15_face - Item name changed
item_research_jar_15_head - Spawns a jar containing an alchemist's decapitated head
item_research_jar_16_claw - Spawns a jar containing a claw of a now-extinct lizard
item_research_jar_17_frog - Number seventeen, frog inside viscous jar
item_research_jar_18_tentacles - Spawns another jar of sea tentacles
item_research_jar_19_heart - Spawns a jar containing a, still beating, heart
item_research_jar_20_intestines - Spawns a jar of crocodile intestines
item_bonesaw - Spawns the TF2 Medic's melee weapon

Giblets. - Replace # with: arm_left, arm_right, leg_left, leg_right, torso, head, or pelvis
item_giblet_citizen_man_head
item_huntsman_#
item_woodsman_#
item_goatman_# - Extras: horn_left, horn_right
item_fishdog_# - No pelvis or leg_left
item_crowman_# - No arm_right or leg_right. Extras: foot_right, hand_right, upperarm_right
item_crowking_# - Extra: hand_right
item_batbarber_# - No pelvis. Extras: hand_right, tail

These are meant to be used with the spawn command.

door_swing_metal - Spawns a metal door
door_swing_metal_grate - Spawns something, might be invisible
door_swing_wood - Spawns a wooden door
door_swing_wood_shack - Spawns a shoddy, silent, breakable wooden door
physics_axe - Spawns a Woodsman's axe
physics_barrel_wood - Spawns a barrel
physics_barrel_explosive - Spawns an explosive barrel
physics_crate - Spawns a crate
physics_crate_metal - Spawns a metal crate
physics_crate_ammo_pistol - Spawns a bullet crate
physics_crate_ammo_shotgun - Spawns a shell crate
physics_crate_medical - Spawns a crate of vitalizing tonics
physics_luggage_small_1 - Spawns a small handbag
physics_luggage_medium_1 - Spawns a medium white suitcase
physics_luggage_large_1 - Spawns a big brown suitcase
physics_pickaxe - Spawns a pickaxe
physics_sack - Spawns a sack
physics_shovel - Spawns a shovel
physics_book - Spawns a book
physics_boot_1_left - Spawns a boot
physics_knife_wood - Spawns a knife
physics_fork_wood - Spawns something, (indicated by the console saying that) but nothing actually appears
physics_cup_wood - Spawns a cup
physics_mug_wood - Spawns a mug
physics_plate_wood - Spawns a plate
physics_torch - Spawns a burning torch
physics_hatchet - Spawns a hatchet (it's what The Goatman throws at you)
physics_mop - Spawns a mop
physics_pillow_bed_# - Spawns pillows. Replace # with numbers 1 to 6
physics_razor_barber - Spawns a gigantic razor
physics_crowbaby_egg_spawned - Spawns a Crow Crook egg which hatches if near you

These are meant to be used with the spawn command.

ai - Spawns all the ai in a stack
ai_huntsman - Spawns an unarmed Huntsman
ai_huntsman_armored - Spawns an unarmed armored Huntsman
ai_huntsman_armored_rifle - Spawns an armed armored Huntsman
ai_huntsman_armored_torch - Spawns a torch wielding armored Huntsman
ai_huntsman_rifle - Spawns an armed Huntsman
ai_huntsman_torch - Spawns a torch wielding Huntsman
ai_woodsman - Spawns an unarmed Woodsman
ai_woodsman_armored - Spawns an unarmed armored Woodsman
ai_woodsman_armored_axe - Spawns an armed armored Woodsman
ai_woodsman_armored_torch - Spawns a torch wielding armored Woodsman
ai_woodsman_axe - Spawns Jack from The Shining, or the Psycho from Cry of Fear
ai_woodsman_torch - Spawns a torch wielding Woodsman
ai_merchant - Spawns the merchant
ai_rat_# - Spawns rats. Replace # with black, brown, gray_1, gray_2, or infected
ai_crowman - Spawns Crow Crooks?
ai_crowbaby - Spawns baby Crow Crooks?
ai_crowking - Spawns a king Crow Crook?
ai_batbarber - Spawns Bat Beasts
ai_fishdog - Spawns aquatic dogs
ai_goatman - Spawns The Goatman. ai_goatman_bloody spawns the injured one
ai_spoiler - Spawns the Dillon Pillar

These are meant to be used with the spawn command.


corpse - Spawns a stack of every AI in the game and they all fall through the floor
corpse_woodsman - Spawns an unarmed Woodsman that falls through the floor
corpse_woodsman_armored - Spawns an unarmed armored Woodsman that falls through the floor
corpse_huntsman - Spawns an unarmed Huntsman that falls through the floor
corpse_huntsman_armored - Spawns an unarmed armored Huntsman that falls through the floor
corpse_batbarber - Spawns a Bat Beast that falls through the floor
corpse_goatman - Spawns a Goatman that falls through the floor
corpse_fishdog - Spawns an aquatic rottweiler that falls through the floor, while playing its death animation
corpse_watchdog - Spawns a dog shaped camera that plays its death animation
corpse_crowman - Spawns, not the Dillon Pillar, but a t-posing Crow Crook that falls through the floor
corpse_crowbaby - Spawns, not the Dillon Pillar, but an actual Crowbaby corpse that doesn't fall through the floor
corpse_crowbaby_egg - Spawns a hatched Crowbaby egg that floats
corpse_crowbaby_egg_spawned - Spawns a hatched Crowbaby egg that doesn't float
corpse_crowking - Spawns a t-posing Crow King that falls through the floor

suggestions for crap to add is appreciated
or if commands are mislabeled or don't work
oh and remember to type "quit smoking" into the console!