My experience is more with applied math (for physics and engineering), which is kind of another world from what mathematicians do. I'm not sure which you would prefer, but if it's the former I recommend checking out the table of contents from Mathematical Methods for Physics and Engineering by Riley, Hobson and Bence(can be viewed on amazon). It has a nice listing of all the topics, mostly in the order you'd want to study them (with a few exceptions). Doubt I'd recommend it as a first book in those subjects for an 11th grader, but it should give you an idea of what's out there.
http://www.amazon.com/Mathematical-Methods-Physics-Engineeri... (click to look inside -> table of contents)
The "other world" that I'm much less well read in more involves proofs and foundations, and would probably start with Euclidean Geometry and Set Theory, then topics like Abstract Algebra, Topology, Advanced Calculus, Real and Complex Analysis, Category Theory, and so on.
I'd also recommend j2kun's blog, which I've found great for a first intro to many topics, though it has a bit of a programming focus: http://jeremykun.com/ As mentioned above, Khan Academy, Coursera, Udacity, EdX and the others are good too, especially since they're free, so you don't have a lot to lose if you get in over your head.
Not sure if this will help, but it can't hurt.