A famous Chinese proverb says that a journey of a thousand miles begins with a single step. Well, the same is true with the road to becoming a physician.
While in high school it is important to get a good general education. This is good by itself, but also prepares you for doing well on the SAT. Taking a complete science curriculum in high school is key to having a good science foundation for future science courses in college and medical school. Take four years of science in high school, including biology, physics and chemistry, plus four years of math (including calculus). It is so important to have a good foundation in these sciences so that you can learn the subject matter in college well enough to earn A's. It seems inconceivable that someone can do well in the killer science courses in college without having been introduced to these in high school.
Getting high grades is the most important component in being accepted into a prestigious college. However, do not be tempted to take easy courses that allow you to get high grades. This will not prepare you for taking courses in college that are much more difficult than those in high school.
More importantly, high school is the time to learn about the real world of medicine (not the one you see on TV). Do something that gets you in touch with the medical field to see if it is right for you now, before putting in all that effort. Learn about other fields that may interest you so that you may compare them to medicine. Also, for the same reason, take elective courses other than sciences to see if these interest you more.
When I was in high school, I volunteered at my local hospital. I enjoyed my experiences there, especially in the emergency room. This was part of the impetus in my decision to become a physician. Many years later while in medical school, I selected emergency medicine as my specialty. I then completed an internship and residency in emergency medicine and have been practicing emergency medicine ever since. It is surprising how small decisions at an early age can influence the rest of your life.