It depends on what you're looking for in a scent.
If you like classic, more grown up (but not too grown up!) scents, I happen to love Tommy Girl and Vanilla Fields. There are tons of scents in this category though. These are the only ones I've used that are classic.
I also happen to love Life by Esprit (a light fun, yet grown up scent) and Body by Victoria's Secret (possibly the best sexy, musky scent I own...I don't like musk, but I love this one).
Clinique, Calvin Klein, Liz Claiborne are some that I've heard are popular, but that I haven't tried.
A great place to go is Sephora...they have a teen+baby frangrance section with some really nice stuff. I really want to try Olily's Orange Stripe, Hugo, Baby Doll by YSL, Pink Sugar and the Fresh Sugar, Comptoir Sud Pacifique, and Demeter fragrance collections. I better get started!
Teens wear pretty much anything they like. I would just sample around until you find what notes you really like and then buy fragrances with those notes. (I happen to love citrus, apple, cardamom, nutmeg, vanilla and light musks.)