I've experienced something similar to this before. Last year I was part of a mini-concert and I was required to play any guitar song of my choosing infront of an audience of roughly 40 people. Prior to my performance, I was nervous as hell and constantly asked my teacher on what would happen should I screw up. In the end, I did not make any mistake and I ended up being happier. My best advice would be to imagine that you are the best player of that song and smile all the time. Also, you can look at your instrument/music sheet too, that makes you seem alot more focused on the song. If you make any mistake, just give a small laugh and either apologize or continue as if nothing happened. Worst thing you can do when you mess is up think "Oh [bleep] and freeze with the nervous look on your face.