From a female person who also has a birthday in December: Whatever you get her, make it NOT something from the “********* Gifts” section at your local big box store. At 13, you can’t go wrong with a well-bound diary with a lock, and a selection of colourful gel pens. If she’s a fan of any particular shows/games/bands/whatever, anything related to that would be cool – I’m thinking throw blankets, pillow cases, t-shirts, etc.