My daughters 13. Depending on her personality, I would say makeup, jewelry (doesn't have to be expensive), nail stuff like polish, files, glittery kind, artsy stuff to add to it. They love gift cards. If you don't know of CD's she would like maybe a gift card to a music store. A diary. Yummy smelling lotions from the mall. A cute purse or makeup bag. Artsy stuff if she's into that. Hair accessories, straightener, curling iron, etc... Hope that helps!
I'm thirteen, and what I really wanted for my last birthday was only a few shirts because my old ones were too small... But teenage girls like electronics, gift cards, movies (the Hunger Games is REALLY popular right now), apparel, and really just about anything! Also, we really don't mind being asked what we want for our birthdays, so I'm sure she'll tell you.