just relax. The reason most...or at least a good number of people, can't (or think they can't) dance is cause they think too much about it, can't keep up with the music, and look awkward. It's pretty much, if the song makes you want to move, then move with the song. Look at stage dancers during concerts, single just ONE out and hit the mute button (preferably, not the HOT one or you'll be distracted). Doesn't look all that coordinated eh? you're not gonna be lookin like a pro, but then again, 99.9 percent of people don't look like pros either. Just relax a bit. Remember the point is to have fun. If you're not confident and all worried and crap, you're not havin fun. You're goin to a dance with a hot girl, don't kill it. If you want a hobby tho, I recommend learning to spin fire or rave with glo-sticks. In high school, that kinda compensated for any lack of ability to dance...then again, you have to kinda like techno too. On the up side, hand-eye coordination skyrockets and that'll help you frag in CS.
Oh, and you forgot the golden rule....pics? I expect many since a dance is a good excuse to bring camera.