Just about everything you normally take in high school can help prepare a strong application for law school. That's because just about everything you take in high school requires some sort of reading, analysis or writing. For your application to law school, you'll need the following:
1. An undergrad degree (BS, BA). You can major in just about anything, but keep in mind what you want to eventually do as a lawyer. If you want to be in patent law, you might want to major in engineering, physics, etc. If you want to be a general trial attorney (litigator), speech, debate, or communications might be nice. Given that a good portion of law is created or argued based upon public policy considerations, a background in political science and public policy might be a great way to go. Regardless of what undergrad major you eventually choose, do well with your high school grades so you can get into an undergrad school that specializes in that subject.
2. A good undergrad GPA. Whatever major you choose in undergrad, do well at it. Your selection chances for law school are weighted heavily on both your UGPA and LSAT scores.
3. A good LSAT score. You'll take the LSAT just prior to applying to law school. It tests your logic, analysis, and reading skills. Right now, learn to read and analyze everything with close scrutiny. In fact, the more boring the subject is, the better. You get this opportunity every day in high school. The LSAT has a reading comprehension section that contains four passages; and the topics can be fairly mundane. Also, you want to develop good reasoning skills. Any classes involving math, philosophy or debate is helpful.
If I could start high school over again, I'd probably elect to take more speech, debate and philosophy classes. Also, make sure you maintain your social development throughout high school. You'll see some people who study so much, they don't find the time to have some fun and make friends. These people usually get into good colleges, but will end up working for people with more substantial social skills. Learn how to interact socially with people from all backgrounds. By understanding what motivates different people, you'll be a far more effective lawyer.