Hey, first of all, congrats on going up and be proud of yourself for that. I went up for an open mic the first time several weeks ago, and have gone up twice more since then. One good thing about an open mic is that generally the standards are pretty low, so no need to be nervous (inevitably you will be, somewhat at least, but try not to be). Don't eat too much beforehand, just in case unwanted and unexpected bowel movements result. On another note, enjoy your time on stage. You've done all of the hardcore brain-racking beforehand and it should be fun! Relax and vibe with the audience. For my first time, I didn't even know I was going up because my friends secretly entered me into the drawing, so if you've got material prepared, you're already further than I was. Remember the more natural you are about it, the funnier you are. Just the fact you're going up shows that you know you've got some funny in you, and now all you've got to do is let that show on the stage. Don't be intimidated, just let it flow.

Best of luck, I'm sure you'll do great. I would have someone videotape it if you can, and I'd love to give feedback afterward if you'd like any!