As Juulie said - pregame. Then pregame some more. (And not the kind involving a six-pack!)
If you're comfortable with it, mental rehearsal might help. Visualize yourself going out there, making a few calls, getting things right. Ignore the visions of your pants splitting in front of the cheerleaders.
Since it's your first "real" game, try to settle into a comfortable rhythm early. Don't be afraid to call the first foul/violation that you do see. At the same time, don't get nervous if you go a few minutes without calling anything - it simply might not be there!
I haven't done it in a while, but I found that listening to some nice calming music before a game can help, too. Just don't stick in your iPod while your partner is talking during the pregame!
Best of luck and let us know how it went.