Stop sweating; you've got the gig.
We get nervous about our first gigs. We want to make a good impression, get invited back, and impress. But we forget that we had to do those things to get invited to play in the first place.
Relax, you already won over someone enough to get invited to play.
Now, you must show up, do the hard work, step into your fear, and get invited to play again tomorrow.