What are the requirements to get into Harvard Medical School?

I am just planning ahead and i was just wondering what kind of grades do i have to have?? When do they begin looking at grades and stuff?? I want to be as pediatrician. And how can I get a full scholarship??? I know I am very young to start thinking about this stuff but I really wnat to take care of little kids and stuff. That is why I am startign babysitting(i am takign the red cross course maybe). I can't wait to see what peopel can tell me. Thanks!!!!!!!