Patrish--Find a song you like, that you know pretty well, so you won't be nervous. Wait until you've got some more confidence and experience; then try out some some songs you've recently added to your repertoire..
Now pitch the song you've chosen in a key that suits your voice. If you want to experiment beforehand, try singing it in different pitches until you find one that's neither too high nor too low for your voice (don't worry about thte people who join in on the chorus--they'll figure out a register or harmony that works for their own voices).
If you wish, you can check your pitch against a piano or pitchpipe or some other instrument. Figure out the note the songs starts on: Now you know what note to ask for before you start. Or if you sing a cappella--just start, and if it's a bad key for you, just stop and start over again--it's perfectly acceptable in a cappella song circles.
--Charlie Baum