People wear different kinds of clothing -- usually camoflauge -- but that's up to you. I would suggest shoes/boots with ankle support. The terrain at the field is hilly and there are lots of ways to twist an ankle. Paintball gloves are really handy too (no pun).
There's water at the field, but it's a good idea to bring gatorade, 10-k or whatever you need to keep yourself hydrated. Buddy and the crew are great, but I don't think they'll give you mouth-to-mouth if you forget to drink water
The field equipment is pretty good -- most (if not all) of the masks have thermal lenses and the marker of choice is the unbreakable VM-68.
Leave your paint at home -- this is field-paint only (usually RPS Stinger, aka BigBall)
That's about it.