What you will need is a Mining Hat. It is quite obvious cause it emits a light for helping you see without use of torches, or drink a Shine Potion, it'll emit a aura of light for 5 minutes so use it wisely. Also you will need a pickaxe and a sword for protection and mining. You will also need a Magic Mirror to return home instantly.

You will need: Rocket Boots/Spectre Boots or Wings. These are useful for flying around places that you cannot reach. What else you will need: Water Walking Boots/Obsidian Walking Water Boots and Lava Waders. These are useful for walking on water, lava and fire blocks, you are immune to fire blocks with a Obsidian Skull or Obsidian Walking Water Boots, walking on lava with Lava Waders or the Lava Charm can save your life if you are miss a jump or being knocked out by an enemy.

You will need: Spelunker Potion, Shine Potion and Health Potion/Mana Potion. Spelunker is VERY useful for searching ores that you were looking for ages, once again i mentioned Shine Potion in the equipments tab, it emits a aura of light for 5 minutes and the health/mana potions are useful if you are about to die.

If you have coins with you... You might have bad luck into losing them if you die somewhere very very deep. Always put your coins in a chest or piggy bank before you go mining!

Thank you for reading this guide, i hope you learned something out of this. Also, good luck mining out there!