Firstly, DON'T BE AFRAID. There is nothing I hate more than bieng teamed up with a coward afraid of being hit. If I'm in a pinch and need cover, I want to know my buddy is there shooting back, not running away. Besides, it doesn't even hurt that bad. It's similair to being hit with a dodgeball. It's meant to be fun, which it can't be if the balls hurt too much.
Second, take good shots. Don't go wild and start shooting like mad, but neither should you only fire three times during the game. Before you play, fire about 20 rounds to determine the general distance and accuracy of your gun. It's typically about 75 ft. If you're going to a field, they'll loan you the standard Tippmann 98. It's ideal range is between 50 and 75ft. but it is effective up to 150ft, if workign properly (it's a loaner, so that may vary). If you know your limitations, you know how effective you can be. Also, bear in mind how much paint you have at your disposal, and use it accordingly.
Third, be quick. Don't hold still too long, don't linger in one area, and keep your rate of fire high (I prefer a double- or -triple tap: firing two or three rounds in quick succession, then adjusting my aim based on those shots, and repeating until the target is hit or out of sight). Speed is your friend.
Fourth, use cover. Hide behind the objects. Also, bear in mind that your paint can be your cover. If you need to run, shoot rounds off in the general direction of the opponents while you run. They don't want to be hit eihter, so they'll hide.
Fifth, have fun. Don't get stressed out or nervous. Just relax and enjoy it. It's a great form of entertainment, and provides hours of good conversation material.
Also, something I've learned over the years: wear a hat. Wear it backwards. It hurts (a lot) when you got shot in the back of the neck. Put your hat on backwards, pull the brim down over your neck, then put on your mask. Also, try to wear a sweater with a collar to protect your throat.