If you're playing with more experienced players, learn from them. Ask them before the game what you should be doing, and how to be playing the field. After each game, get some feedback off them - what did you do well, what could you do better?
In general - talk, lots. Shout out (loud) everything you see going on. Your teams loses, your kills, where people are, where they're shooting etc, etc. Most teams use G1, G2 etc for kills and O1, O2 etc for loses - but give details - G1 1 Dorito 2 is much more useful than just G1!
Keep your head up as much as possible, and play tight to the bunkers - really jam up against them - so you offer a small target. Try and be up, shooting, as much as possible, but don't get over eager and stay calm - lots of people hit their bunker and snap straight out into paint!
Above all, enjoy it. Don't get frustrated and hung up if you don't win games, just have fun. Oh, and if you get the chance, bunker people. Even if you lose, bunkering people is fun...