If you are not yet 14, I would encourage you to look into MathPath. If you are up for a real challenge, and are at least 13, check out Canada/USA Mathcamp. Admissions are very difficult for Mathcamp, but as a girl you will get some slight preference.
My kid has done both (multiple times for Canada/USA). I could not recommend them highly enough. For some kids, the experiences have been life changing.