In high school, take challenging classes. Med school WILL be very difficult & time consuming, so best start preparing yourself early.
Take AP, IB, or honors courses, whatever ur school offers. Do it in any subjects ( English, another language, Statistics, Calculus, History, Sciences) I'd definitely recommend AP math. Be sure u can handle it, though. Do no go overboard. It is okay to gradually challenge yourself further. It is better to do well in a few AP than mediocre or poorly in a lot.
If u r thinking of being a biology or chemistry major in college, taking AP Bio or Chem may not be as helpful as u want. You'd have to retake biology or chemistry in college, so even if u scored high enough on the AP examination, u will not get college credit. Yes, this means if u score at a certain level on the AP exam (usually 4 or 5, it is out of 5), u can get college credit which means u r excused from taking it in college. BUT, taking high school AP does better prepare u for college science. I know this!
Also, consider courses like nutrition or athletic training/human anatomy, if u want. I'd say honors/AP/IB is more important, because u will more likely forget what u learned in these courses by med school.
Another note. do take ur time to be a more well-rounded individual. Med schools love to see someone fresh & different, yet still driven in medicine. If u have certain talents or passions, such as a sport, cooking, philosophy, writing, or music. do not hesitate to take a course on it! It is NOT just all about science!
I would recommend if necessary to take something to improve writing skills & public speaking ability. That helps tremendously. Language skills (grammar or speaking a foreign language) is incredibly helpful for medicine.
Here r some of the courses I chose to do throughout high school:
-took two languages . Latin & Spanish
-AP English, Histories, Biology
-biology (I took both. AP was better prep), chemistry, college pre-calculus, physics
-as many honors as I could before AP
-Athletic training
-Media arts
-A class on typing/using computers
That is all I can remember. I was very involved in extracurricular activities, so remember that too! (hospital volunteering, leadership positions, yearbook editor, etc)
I hope this helps! Been there done that in the past few years.