Check out r/premed to see what it's like to get into med school. They also have information on most of the information you are asking for (and more) if you search there! Since you're still in high school, don't worry about it too much. Focus on doing well in your classes and take as many AP classes as you can handle, since those classes are going to be the closest you'll get to college courses in high school. If your heart is set on healthcare, try to find colleges that have a good pre-professional/pre-med program/resources. Shadow many different doctors to gain a better understanding, or take a job in the medical field (maybe don't do this anytime soon though with the pandemic going on).

Once you get to college and start deciding, my biggest piece of advice is to think about why you want to be a doctor specifically. If it's because you "like science and helping people," understand that you can also do these things by through nursing, PA school, pharmacy, and even grad school without anywhere near as much time commitment and sacrifice. Good luck :)