That's a gig.
Get there early. That way you have time to set up carefully, making sure everything is working properly, and maybe play a few songs for a soundcheck.
Bring some tools; pliers, wrench, screwdrivers, etc... Without fail your drummer will not be able to find his drum key.
Bring wirecutters in case you break a string. And bring extra batteries. BRING DUCT TAPE.
Bring extra strings, and guitar cords. Strings break and cords go bad at the worst times.
Do not play louder than the singer. Get a good balanced sound with the singer on top.
Make sure everyone tunes up carefully 10 minutes before you go on.
Do not get stoned.
The hardest part is just relax and have fun. Don't sweat it if you make mistakes. You will.
Rock out Petrix!
If you know something better than Rock and Roll, I'd like to hear it - Jerry Lee Lewis