4 years college
4 years medical school
3 years pediatrics residency
3 years pulmonology fellowship
Medical schools do not require you to major in biology or chemistry, although biology (especially physiology and biochem) is particularly helpful in hitting the ground running as a first year medical student. I have had a fair number of medical school friends who majored in English, history, philosophy, etc. and it is my belief that admissions committees like the non-traditional majors.
If you wanted to become, for example, a pediatric cardiologist, you would still have to do college, medical school, and pediatrics residency, but you would instead complete a fellowship in cardiology.