Khan Academy. If you want harder pre college problems pirate Serge Lang's basic mathematics and do the problems in those.
Past that you need to study calculus and some math formalities. Pirate and read Apostol's calculus volume I/volume II and supplement with youtube videos. Buy the Book of Proof and read it.
Two things:
1. Take organized and good notes
2. Comprehend ideas more than pure problem solving
3. Have fun, talk and spiel to people about math, and don't be afraid to take breaks or watch fun stuff like numberphile.
Once you do this, the sky is the limit. You can find any subfield and enjoy studying it. Just google for introductory texts of it.
