Depends on the girl...some girls need a month to find a dress, some need a day.
Prom lick nuts, unless it's a nookie run and you know you are going to get some, don't bother going.
(The other reason to go is if it's your girlfriend, then it's worth going with her, but if she was your GF you wouldn't have to ask, so I'm assuming that's not the case)
Find some girl you like, and ask her out on a normal date, go do something that the two of you enjoy. It will be a night much better spent. Not that I can I minded seeing the girl I like head to toe in a tight black velvet dress, but I think I'd rather have asked her to go to a movie or go fly a kite or something (she loves flying kites, she never got to do it when she was little).
As for dinner reservations and stuff, well it depends on your city and where you want to go. I rented my tux about 2 months in advance, and we made our dinner reservations about a month in advance, but there were 12 of us in a group, it wasn't just me and her, and it was one of the nicest restaurants in the city (small town, no really big fancy $500/meal restaurants there).