Welcome to CC.
What math courses have you taken? AP Physics C (especially the E&M portion) is generally considered the hardest course at most high schools. Familiarity with calculus would be very helpful but not absolutely necessary (Newton invented calculus after all).
Theoretical physics is probably the hardest subject there is (I may get some arguments from some mathematicians). Theoretical physicists are working on the thorniest and the most difficult problems in the world: to explain and predict how everything works in the universe, accurately and consistently.
The best books to learn physics, IMO, are still the The Feynman Lectures on Physics. Feynman is one of greatest theoretical physicists of all time. He had a knack for figuring out and explaining how things work physically and intuitively, without relying on a bunch of formulas.