I would start by reading psychology books and really understand human development. Starting with birth all the way to death to really understand how and when the brain matures as time goes on. I majored in psychology and it was a lot of fun, it would help by watching videos on human development and reading books. By doing your own research, you might find what you want to specialize in and what age group of patients you would want to work with. Hope this helps
You'll have to go through medical school like every other doctor that specializes in something. I would recommend your undergraduate work be a major in neurology or psychology, and that you be sure to take and get a good grade in the relevant science classes: organic chemistry, physics, and calculus.