Get good grades. You'll need good academic marks to get into a competitive premed program, and while your undergraduate grades matter more to med school admissions boards, they'll look at your high school grades, too. A strong record of academic excellence will go a long way.
Take all of the science classes that you can, especially Advanced Placement (AP) courses. Take as many biology, physics and chemistry classes as you can; if you can take advanced topics or anatomy, even better. Many schools offer AP courses in one or more sciences, too, so make sure you take any prerequisites early so you have time to take as many advanced courses as possible.
Take advanced math and English courses, too. An M.D is a research degree; you'll need strong writing and reading comprehension skills to survive the rigors of med school. A strong foundation in math will only help you with your science courses as well; calculus isn't required, but it is recommended.
Take a summer job, internship or volunteer position in the medical field. Any experience and interest you can demonstrate will separate your application from the pack. Talk to your guidance counselor about opportunities for volunteer or summer positions in local hospitals or doctor's offices.
Practice your test-taking skills. The path to and through medical school goes through a barrage of standardized tests, and the better you are at preparing for and taking these tests the better your chances of success.
Research premed programs and make sure you meet the requirements for schools you're considering. Don't let the lack of a foreign language requirement or extracurricular activities keep you from being considered for your first-choice programs.