It is really great that you have figured out what career you want to pursue. This is a pretty huge and often difficult decision.

Did your parents give any explanation for their reaction to your decision? Trying to better understand why they reacted this way will help you to know how to convince them and win their support.

Maybe they don't think you know what becoming and being a doctor entails. If so, doing research into the profession will help you prove to them that you know what you're talking about.

Consider talking to your own doctor, or friends whose parents are doctors, to get more insight. You may also want to get a part-time job or internship related to the medical field to get even more exposure.

Or maybe your parents think of doctors in a certain way and don't see you in the same light.

In any case, perhaps you should remind them of their promise to support you in whatever you chose to do. It's possible that your parents will come to support you if you continue to show them you are serious about your dream. In the meantime, doing well with your schoolwork now may qualify you for scholarships in the event that they don't back you financially during this process. (If you are talented enough to qualify for a scholarship based on merit and not just need, that would be great too!)