Honestly, I always found that the onboarding to Battletech has been pretty horrid across all games for people who aren't familiar with the universe. I had originally written this for a guy on reddit who was struggling but I somehow couldn't post it and didn't want this to go to waste. I hope this guide helps all the newcomers. Here are a couple of core rules I learned for myself, they should keep you alive, help you with early building experience and needs and just give you the time and survivability to refine things from there for yourself (and also work in the Mechwarrior games for the most part). Most of these still form my own guidance even after 2K hours in the game:

• Start a build with max armour. Always go full armour, then fill the mech up from there and then shave of a few points to not go overtonnage. Armour is akin to a shield in this game and cheap/fast to repair, structure HP is in essence you being on your last leg and opens you up for critical failures, so don't let it come to it. Also by pressing SHIFT while changing armour values you can assign single points instead of 5 points.
  
• BUILD HEAT NEUTRAL. This is absolutely essential and you can even overcool a little but with time you can go more risky (or on mixed range builds plan for each bracket's cooling only). This will never really change though as you likely won't redesign heat sinking for each planet and especially in mods which have ammo cook offs on overheat the heat needs to be under control.
  
• Plan for a mission to take 12-20 rounds. So a good average is to bring ammo equivalent of roughly 16 rounds. This means you won't run out of ammo but also won't waste space bringing too much. With time you can refine this further and on mixed range builds this would become less as guns are only active in some turns, but this should give you a good basic to work around.
  
• DO NOT PUT AMMO INTO THE CENTRE TORSO OR HEAD. With time and bigger mechs (and CASE) you can take more risks but putting ammo into the cockpit practically guarantees a pilot death when hit and the CT is the location of the fusion engine, an essential part to your mech and pilots life (big boom underneath your butt won't help with survivability either). Usually, legs can be a good spot for ammo. Side torsos are next. Arms I would normally avoid but this can be an option if the gun using the ammo is in it and you otherwise cannot find space - won't need the ammo if the gun is gone and vice versa after all.
  
• For a starter I definitely recommend to focus on a single range bracket (Long (LRM/Gauss) / Long to Medium (Ballistics or Lasers) / Short to Medium or vice versa (Medium Lasers/SRMs)). Personally, I find mixed range/jack of all trade builds to offer little in individual killing power and you cannot always target a specific enemy with all mechs.
  
• Lasers are good beginner weapons. You won't run out of ammo, they are good and strong and you just worry about your heat.
  
• Last building tip - go with the flow. Each game is going to be different as you encounter different mechs and have different weapons available. I almost entirely avoid the markets, mostly buying only ammo or one specific gun. That being said I also avoid building mechs around weapons I cannot replace - loosing that one Gauss Rifle would immediately force an entire mech rebuild so it is probably best to wait until you have a spare one in stock.

I may extend this section down the road. Please be aware that I am now almost entirely playing with mods, so this is just a general advice and may apply more once you step into mods:

• Focus your pilots on a role and spec them for it - once again, Jack of all Trades type pilots are ultimately nothing special and in most mods using the same mech gives you affinity for it so that would additionally boost you. Ultimately though this is really one for you to figure out over time - pilots suck at the start and become amazing as good mech builds keep them alive even if their skills aren't perfect so you have room to test things out.

Personally I have created some roles for myself that I tend to build around both in terms of skills and mechs but be aware that this may be mod dependent:

• Commander (Movement + Sensors) - Mostly a role I use in mods but it gives you extended visibility and a character to sensor ping.
  
• Heavy Assault (Survivability + Accuracy) - This is your mech to face tank damage. Alternate who takes damage and which side of the mech takes damage to max out your survivability.
  
• Flanker (Survivability + Speed) - Faster mechs that go around and side strike or even get into the back.
  
• Recon (Speed and Sensors) - Oddly this one prioritises speed and only gets Sensor Ping as this is usually your light mech drawing fire.
  
• Sniper (Accuracy and some Survivability) - This is your range monster built around Ballistics or PPCs/Large Lasers only. the survivability just helps getting it into position or to deal with an exchange of fire.
  
• LRM Boat (Split Fire and Heat Control) - This is your man to just pepper the enemy, strip their armour (and evasion in vanilla) before the main encounter, mess with their stability and finish of those that show weakness.

This is a smaller section - you learn this over time but here are some general guidelines:

• Focus fire enemies. - "GUNS OFF THE TABLE" is your primary concern over loot, flamer shenanigans or fancy melee duels. I cannot stress this enough, having seen people struggle with the concept in World of Warships and other games this needs to be your main guideline - a mech with no armour but all guns does the same damage as one that is fresh out of the factory. Finish them off and remove the threat.
  
• Speed and correspondingly Evasion - These are a free form of armour, anything that misses you doesn't cost you repairs or lives. Knife fighter builds like the Firestarter can exploit this by effectively getting run distance while still shooting, just bear in mind that this also puts you into the melee range of the Atlas you are annoying to death. This becomes more essential in some of the mods, in BT Vanilla and Extended though it is ultimately an armour heavy focus but that doesn't mean just stand around.
  
• Height = Accuracy - Not much else to say but as an added bonus that also means this functions as a counter to the enemy's accuracy.
  
• And a smaller advice, tanks don't like being stomped on or flamers.

The rest you can learn from there yourself.

Stick with the game, it is amazing and can in theory indefinitely be replayed due to the random loot and massive mech volume in the modpacks. If you do and start hitting the mods (SERIOUSLY RECOMMENDED) here is the recommended order of the 3x main ones so you can slowly but steadily increase your skills:

• Vanilla Game - Just start here and learn the ropes, it offers plenty and is a good early playground and avoids you getting overwhelmed in the mods.
  
• Battletech Extended[discourse.modsinexile.com] - A Vanilla plus type mod but with a seriously massive plus to it offering the Clans, various time settings and of course the whole Inner Sphere map.
  
• BTAU[www.bta3062.com] (previously 3062) - This will majorly expand your building abilities and the use of evasion but pretty much all guidelines listed above still apply. I find this to be the best mod (personal preference) offering a massive extension of the game and its mechanics without sledgehammering you into the face. I wish it wouldn't mix timelines but you can find a corner of space for yourself where this isn't noticable so not the biggest worry.
  
• RogueTech[roguetech.fandom.com] - This is the sledgehammer to the face mod. Enjoy :) It has a ton of new mechanics (especially in EWAR concepts) and massive timeline extensions but this also comes with the learning curve of a cliff. My own little Everest, I still haven't really figured it out :) But it is also the most expansive change to the game and biggest content size (I think) so really just something to work up towards.
  
• Last tip but possibly the best thing ever - SARNA [www.sarna.net]is your source for all lore. The wiki is amazing, helps with understanding, learning mechs and what to expect from them (in lore terms, you can always build crazier than the madlads in the lore) and any other questions you could have about the universe.

I hope this guide helped, have fun.
Kill the meat, save the metal.