If you still have long hair like in the photo, go down. I would go with a fancy hair style, as it is prom and it is worth the effort. After all, it only comes once in awhile! As for a dress, I personally love an indigo blue color. As long as it is classy and doesn't have your breasts spilling out of it, I'd probably like the look. Don't forget to wear the most important accessory of all, your smile. ;-)
What is that hair style called when its up, not tall but kind of stacked with like a few strands curled down. Wow... I don't care who you are that is the best... I'll knock caters over to come talk to you... but seriously, what is it called?