You can make that plain navy suit outstanding if you have it properly tailored to fit very well.
For prom, I would go for a formal satin tie-silver or gray, and not skinny. Slim, maybe, say 2 and 7/8" wide, otherwise go with the full 3.5".
White shirt with semi-spread collar. If you go with a shirt off the rack, make sure the cuffs fall to the correct spot on your wrist and that the neck fits correctly-not too tight but not too big. Too big always looks awful with a tie. Barrell cuffs might be the better choice for you, since prom is one night and the shirt will be useful afterward.
Shoes should be black or very dark brown lace ups. Captoes are classic and a great first dress shoe. Alternatively, you could rent some patent leather formal shoes at a local tux shop. This may make more sense than dropping 150 on shoes that you may grow out of. Match the belt, or go for full-on sleek style and wear no belt. Make sure your trousers fit ridiculously well at the waist, though.
Clinch it all with a pocket square to match your shirt. You could get one to match your date's dress, but I don't recommend this.
Now, if you want to be way cool, see if you can add a navy waist coat to your suit, then buy yourself a black bow tie and learn how to tie it.