What can I start doing in high school to prepare to become a doctor?

Your high school classes don't matter other than to get you into undergrad.

At this point, you should be heavily involved in extracurriculars and clubs. You should be volunteering at a variety of charities, not just health care based charities. You then carry this into college and continue as much as possible. Once in college, you should start taking on leadership roles in these clubs and charities, making a difference instead of just showing up.

In college, you should also be involved with research projects, even just as a lab gopher/data collector for the grad students. You won't get your name on any papers, but you will have it on your resume.

Your college performance is what's important for med school. I'm not encouraging you to slack off during high school since your high school performance dictates where you do undergrad, but you have a bit of slack during high school.

Good Luck

Since you are interested in medicine, most colleges will expect you to have taken, in high school, four years of lab science including, specifically, biology, chemistry, and physics; plus four years of math.

Since you may be interested in medicine, it would be a good idea for you to do volunteer work in a hospital or other health setting, as well as in helping-related fields, such as a soup kitchen or homeless shelter.

In most of the US, it can help you, as a doctor, if you speak some Spanish, so it's not a bad idea to take some Spanish in high school or college. This isn't required; it's just good to have.

For going into university, idk if it is the same in your country (if you live in the states) I live in Canada, the first year of university starts kinda like a high school, going to do english, math, and I think science at a university level, my friend told me that at university, they will be doing three sciences at univeristy, chemistry, biology, and physics. If you do not take physics in school, you can do it online (doing a course online) during your school year, if its too much, you can do physics during the summer. Chemistry and Physics have a few similarities on their formulas, but I think I would recommend it if you do not want to get confused on Physics at university.

I know for math, you need to do the higher math, Pre-Calculus, idk what you guys say it in america. Pre-Calculus is a math requirement for university for the businesses, sciences or engineering fields. lmao this is what I know..I hope it kinda helped you.

good luck on your AP's :)