Dear Emmie--At my first open mic I was a nervous wreck. I'd neither sung nor played my guitar in public before. For my first song, I did Peter Paul and Mary's "Leaving on a Jet Plane" and REQUESTED that folks sing along. It's a 3-chord song, and while everybody seems to know the chorus, about 50% of everybody knows most of the verses too. It helped me tremendously!!! (John Denver wrote it, but I think PPM made it famous).
Good Luck, and Have fun!
V