How formal do you mean by formal though? I just mean that lots of sparkles might be overkill if it isn't too formal.
I think, if you like bright colors, that could be a nice, summery fun option! If not, there's the classics like black and white that you can dress up. Strapless can be pretty, but I know I always have to keep adjusting strapless things when I wear them. A halter or spaghetti straps can be more comfortable.
I don't know how much money you have to spend, but Charlotte Russe or Forever 21 can be good for inexpensive ones.... if you want to spend more money, I like stuff like Macy's or the little shops in shopping malls. Nordstrom's also has some has some dresses in their teen department which are comparable in price to Macy's.
I find what's best is to go and try on a whole bunch of things and see what you like!
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