I'm a high school student who wants to get a graduate degree in math when I'm older. What should I be doing now to prepare?

I've never thought that I would, but I found out that I absolutely love math. It may be because of the challenges that math presents or because of my brilliant teacher, but I now know that I want to study math in the future. Now the question is, how do I prepare?

I don't have phenomenal grades (A in physiology, B in U.S. history, B in English, A in A.P. Calculus, A in art, and an A in my sport), but I'm willing to work my ass off for this. What classes should I be taking A.P. courses in? Should I be doing extracurricular activities? Thanks in advance for those who help me out :)