1. Eye protection (full face if you like playing CQB)
2. boots(something that will protect your ankle)
3. gun + battery/charger
4. vest (make sure it can hold the magazine of whatever platform you chose for your rifle)
5. BBs (get well known BBs, a lot of AEG malfunctions are due to bad BBs)
Good to go first game
Only thing to be concern about is whether or not you're gonna go outdoor or indoor. Shooting at 350+ will keep you out of most indoor arenas. The Ares brand has quick change spring which can be a huge benefit. You can have one rifle that's good for both arenas without spending too much time changing springs.
And a just in case....fields will chrono with .20g bbs so whatever your fps in w/e wait won't matter but that one.