Most important is full-seal eye protection, a good pair of boots, and an effective set of camo that matches your playing area well.
Anything else, get it when you need it. The last thing you want to do is buy and haul around a bunch of unnecessary gear. If you're on the field and run out of ammo a lot, then buy extra mags and some way to carry them. If you don't quite have the range you want, buy a tighter bore barrel and/or heavier BBs, etc.
What gear you need depends on your playing field, your game types, and your playing style.