I'm not going to dance with her at the prom
Why aren't you going to dance with her at the prom?
I don't know how to dance.
I sympathize with you on not being able to dance but when you take a girl to the prom it is rude not to ask them to dance at least once. Instead of being anxious about your dancing you should have fun and not worry about it.
I'm not afraid of my reputation or being seen as a loser. I don't want her to feel bad on the dance floor, besides if I'm nervous she won't look pretty.
You should take a  dance lesson or two from someone who knows to dance. It's better to know few steps than knowing nothing. That will boost your confidence.