Medical does not train you to be a specialist. The first two years of medical school are a mixture of classroom and lab time. Students take classes in basic sciences, such as anatomy, biochemistry, microbiology, pathology and pharmacology. They also learn the basics of interviewing and examining a patient. During the third and fourth year medical students do rotations at hospitals and clinics affiliated with their school, The rotations will help a student decide what aspect of medicine they want to pursue. From there they would apply for a residency in a specialty, in you case, you would apply for a residency in pediatric oncology. According to US News and World Reports, these hospitals (http://health.usnews.com/best-hospitals/pediatric-rankings/cancer) are the best hospital for pediatric oncology so you may want to apply for your residency at one of these hospitals. Until then, get really good grades and take lots of classes in math, sciences and social sciences.
Good luck! Natalie