Lots of good tips here especially the ones regarding practicing the song.
In my experience I could never stop the nerves beforehand but once I started to play at my first few gigs I realised the audience aren't actually watching me like hawks and scrutinising my every move like I feared.
There's other people on the stage too and they're on your side, and with doing just one song it'll be over before you know it!
I hope it goes well for you, the first one is daunting so good on you for getting up there nerves or not.