Hi, Paige: that's great that you want to teach math! We really need good math teachers and they are in demand. In my experience, those who really love the subject are the best teachers. So, I hope you really love math and love thinking about how to inspire others to do the same. If you are in high school now the best thing to do is to pour as much energy as you have into your love of the subject. Go above and beyond your homework and ask your teachers how you can learn more. Maybe enroll in a college course if they are available locally. So, that's step one - becoming the best mathematician you know how to be.
Next, let's see if you like teaching. As the other advisors have suggested, see if you can do some tutoring. You might be able to do some peer tutoring. That would really give you an idea of what it's like to work with that age group. Also, ask your math teachers if there's a way for you to be a class assistant. As for college, the obvious move is to major in math . . . but at the same time keeping open to other options. Once you get to college you'll have other decisions to make about what sort of math you like best, theoretical or applied, or some other science that uses math as a core skill. There are many good colleges that offer opportunities to become a certified teacher . . . and then, of course, there's grad school to consider. To become a master teacher you may need a masters degree, and depending on your level of interest in math, a PhD. However, we're getting ahead of ourselves here, especially if you are still in high school. For now, exploring further your interest in math and working with high school aged students are two things that will keep you plenty busy. Oh, and don't forget to ask all your math teachers how they got to where they are in their careers. Ask them what they love AND hate about teaching math. It's good to know both sides of the equation, if you know what I mean. Best of luck. (Oh and one resource to check out and something to shoot for might be the Math for America program for your post college years: http://www.mathforamerica.org/)