I'll second the advice that you seek advice from a teacher whom you regard as competent to get started. A high school is pretty likely to have extra books sitting around and it's pretty likely that you can borrow one. If not, they're often available at libraries and so on. There is such a glut of books available at that level that it's hard to know about any one in particular, and my experiences are too far behind me to remember. :-)
Perhaps even more important is that you find someone (perhaps the aforementioned hypothetical teacher; if that can't be done, sensible questions usually seem to at least receive hints on sci.math, although one has to cut through a lot of dross there too) who can give you some guidance while you study. It's all too easy to become distracted and miss the really important concepts. (A well written book should help ameliorate this effect, but it's always something for which to guard.) And while it best to work out as much as possible by oneself (IMHO), it's also silly to get stuck on and become frustrated by something that could easily be cleared up by spending a few moments with someone knowledgable.
Don't forget to do a lot of exercises, too. :-)
