I think I've covered this before so I'll keep it short:
- I need a good game. For me this is basically a game I like, one that starts the wheels turning just by flipping through it. It might be the setting, the concept, the mechanics, or even the art, but something about it has to inspire me.
- I need some players. Interest and availability are the two key elements here. Experience isn't super-important - I can teach you the game if you're interested and can show up regularly and I can provide dice, character sheets, and books for the game.
- I need time. This is often the hardest thing to come by. It's not really a tool but it is a necessity.
Anything else is gravy. You'll probably want some pencils and paper. You'll probably want some kind of character sheet. You'll probably want some kind of dice or cards or coins to flip. That said there are workarounds for all of these - a smartphone can replace all of them, and can probably keep a copy of the game rules available as well. A tablet or a laptop can do the same.