Best of luck!
First point: I've only ever told one person I liked her and she very gently put me down, kind of. I just remember feeling peace afterwards. That was about a month ago
Second point: Ummm, tell the truth, if you just want to be friends, try to tell her in a kind way
Third point: I've never been to prom, but talk to your friend, as I still haven't really talked to my friend, because I have a hard time talking to people, face to face.