Since you are only in grade 9, you still have plenty of time to decide what you want to do. I would recommend taking AP chemistry, AP Biology, and AP Physics at some point during your high school career. Try to get through AP Chemistry as soon as possible.
Also, if you are really ambitious, look up some nearby university professors and tell them you would like to do research with them over the summer. Since you are a 9th grader, they will likely be inspired by your ambition and one of them will almost certainly make room for you to volunteer in his or her lab. If this isn't possible, see if you can start volunteering at a local hospital. This will be valuable experience years from now on your medical school application. It will also give you valuable insight on how medicine is actually practiced.
Make sure you maintain high grades. (Try to maintain at least 90% on everything you do, but realize that grades are not everything. Also, 95% is high -- contrary to what your parents may think.)
After taking AP Chem, try to sit in on some organic chemistry classes at a local community college or watch some lectures on organic chemistry online. It's okay if you don't remember everything. Merely being exposed to this stuff before taking it at the university will give you an incredible advantage. Try to understand a little of the language and the jargon. Learn some simple reaction mechanisms.
Similarly, after doing AP Biology, look for a copy of "Molecular Biology of the Cell" by Alberts online or at your local library. It's a famous biology textbook that is reasonably self contained. Try to skim it over the summer, and make sure you try to follow what is being said conceptually. Don't turn it into the memorization game where you remember a bunch of vocabulary and forget it later. Focus on the concepts. Again, merely exposing yourself to this stuff before taking it at the university level will help you dramatically. (Even just looking at the diagrams and reading what some of the words mean will help!)
If you somehow get through all of this without a problem, start reading any textbook on Biochemistry.
Do well on your SAT/ACT, have a high GPA, nail your college apps by talking about your goal of becoming a doctor and shamelessly plugging how you volunteered over your summer breaks and how you have taken the initiative to study so much more to learn the fundamentals of medicine, and you will likely find your way into a good university.
Then the real studying begins, but since you have seen some of this stuff before, you will have an advantage, and instead of freaking out during organic chemistry like most premeds, you will be well prepared to succeed. (Also, make sure you continue research at the university. Medical schools LOVE applicants with high grades, research experience, and a demonstrated commitment to medicine from volunteering.)
Also, at any point during this laundry list of things to do, if you ever feel like maybe you don't want to be a doctor, that is 100% okay. Don't let your parents decide your future for you. Listen to them when they want you to do well in school, but you do not HAVE to be a doctor to succeed, and if you do not like medicine, you likely won't succeed in it anyway. So if at any point you decide you don't want to do it, switch early and save yourself the grief.
If I had to stress one thing, it is that you show initiative and try to do research or volunteer ASAP. This is the thing that really raises eyebrows and will distinguish you from everyone else. This is also the thing that will demonstrate your maturity and capacity to succeed as a doctor. It will also show you the difficulties involved in practicing medicine. It isn't like TV!
