I want to be a pediatric hematologist oncologist but, what do i major in?

My cousin is a Nephrologist, specializing in kidneys. What I know, I learned from him. It seemed he was in school a lot longer than usual. He said first, all doctors have to get their MD. Once that's done, then they go on to specialize in whatever field they want. Some remain MD's which is our Primary care Docs. Even a Psychiatrist has to have his MD first. So after that's done, you will be guided in the areas you need to study. Of course for what you want to do you will need to study hematology, microbiology, anatomy etc. It's hard work but the world needs dedicated people like you to help children. To many children die of cancers. Good luck