If you don't know him or her well, Disney flicks are a good bet. Keep the receipt in case they already have it and stick with the latest releases as it's more likely they haven't recieved that yet. Gift certificates make great gifts for teens. Choose a music or clothing store that has lots of young people hanging out at it and purchase a gift certificate. Or you can get one for Starbucks, or another popular coffeehouse in your area.
Soaps are always a safe bet when dealing with teenage girls.
Elegant or smelling soaps are perfect for the job. You can add a matching body lotion or shower gel and color-coordinated bath oil bubbles for a gift basket. The Body Shop is a good place to pick up this sort of thing. A Walkman or Discman will be popular with most young people. Just make sure they don't already have the model.