Obviously the essentials. Lots of power outlets for chargers, secure, or moniterd kit areas so things don't "walk away".
Decent washrooms that don't smell like something died in them are KEY.
Ideally a floor made of astroturf, or concrete, just please god not SAND and not GRAVEL. If you absolutly must do sand, don't let anyone shoot any bloody paintballs in the place, because then you get a sandy tar. Some sort of concessions stand would be nice, be it an acessable outdoor BBQ, or some vending machines for quick sugary boosts... Basicly the same stuff you find at most sucessful paintball places, only without paintballs.
As for the field, structures, a nice well thought out urban set up would be awesome. Keep the field sort of unven with few high points that allow for a command of the field. Preferably structures with multiple rooms, and if possible get some old furniture donations to kit out the structures.
Vehicles are cool, especially the enginless permanently parked variety. They add an air of authenticity to a game, and make for great photos. Lighting is also key. If at all, keep it evenly lit across the field, with overall lighting on a dimmer that would allow for multiple light levels depending on the senario. It would also be cool if individual sturctures had self-controlled lighting both Int. and Ext.
speakers rigged up around the perimiter would also be good for ambience. The ability to either blasty some jazzin tunes, or ambient sounds always adds good effect.
Good parking! Please have parking set up in a way that de-kitting a car is discreet, and not in a shady as hell part of town.
I'll post more as i think of it, but thats just off the top of my head.
Just out of curiosity, is this the pipedream of some highschoolers, or do you have legitimate backing? Who are you? How old are you? Before seeing a real reply from the community ,you would need to introduce yourself to them. As it will be stated, many have come here asking what you have asked only to fade back into the wings, while few others have actually opened fields.
Maybe you'll find someone else to help you. Maybe black mesa... THAT WAS A JOKE, ha ha, fat chance.
My Buy/Sell 1337ness rating