The American Girl wiki has announced that the 2013 Girl of the Year's name is Saige and her story will be set in New Mexico. That's all the info available at the moment, when I find more I will post it.
UPDATE: dolldiaries.com has posted that according to the GOTY book summaries available
Saige likes to paint and ride horses.
dolldiaries.com has posted an update that has a small photo of what Saige will look like. She appears to have long brown hair and is wearing a western style outfit and boots. 11/3/12 dolldiaries.com has received photos of Saige in her meet outfit and they are posted on their blog.