When you are a teen with a not-small body, I know how hard it can be going shopping for that perfect formal outfit. The key, however, is to start early. Keep your search broad as well: be sure to look both in your local area and online. You can try catalogs, too. As many avenues as you can think of, you'll want to search. And never let yourself feel rushed. The last thing you need is a calendar or clock dictating to you what you should wear. You won't feel happy with your purchase and it's just silly to spend money on something you don't even like because you were in a hurry.
Next tip: Buy clothing that fits you NOW. Do not buy something too small thinking you're going to lose weight to get into it. If you do lose weight, you can have someone tailor it to your size, but you need as little pressure on yourself as possible as the big event approaches. You should be enjoying the occasion, not dreading it.
If you're a guy, you can likely rent your formal wear if you live in a large city, but make the arrangements as soon as you can so that you don't have any nasty surprises as the event nears. You may be able to rent in smaller towns, too, depending on your size. It just depends on the selection available where you live.
Girls, you may be able to rent your outfit as well (in which case you can follow the route of the guys above and rent your dress in advance), but a lot of young women prefer to have a dress on standby for all formal occasions and will thus wish to buy a dress. Buy accessories that match this dress perfectly, too. That way, you'll never have to wonder what you're wearing tonight.
And let us not forget about your shoes! Go shopping early for shoes, as well, so that you can find the perfect pair to complement your formalwear.