It depends on what type of doctor you would wish to be. If you're talking about a medical doctor, you wouldn't focus on that until you get to college. I would suggest you take as high mathematics as you can. If you want to get your Ph.D. in a certain area that is non-science related, then you don't have a really high need for mathematics.
If you are trying to get your Ph.D. in a science related field, it once more depends on what area you're trying to go into. Since a lot of science requires Physics, you must learn Calculus II at the minimum which you won't learn until you've reached college. However if you have AP Calculus classes in high school, it could be a really good way to prepare you.
Chat with our AI personalities