MIT OpenCourseWare Highlights for High School is also a resource which highlights MIT courses selected specifically to help you prepare for AP exams, learn more about the skills and concepts you learned in school and get a glimpse of what you'll soon study in college.
Overall, you should try to take the most stimulating courses available to you. If your high school doesn't offer courses that challenge you, you may want to explore other options, such as local college extension or summer programs.