How do I tell my parents I don't want to go to medical school and become a doctor?

Becoming a doctor is not a casual decision, most medical schools will not be happy by your age (Serious) when you apply (shows lack of life experience; immaturity; etc.). First, investigate the average stats/age/clinical and volunteer experience/leadership positions/MCAT scores most Pre-Med's have who have been/are accepted at a given Medical school; compare that with your own, and show your parents. They shouldn't be too stubborn to deny numbers.