To become a doctor you first need to attend college and get a Bachelor's degree. It can be related to anything, but most people do something science related (ex: biology), it helps a lot during your time in college if you can get experience working with a medical facility and get good letters of recommendations from your professors. You then need to take the MCAT (Medical College Admissions Test) and apply to medical schools. After you complete medical school, you have to do a medical residency, which is essentially on the job training, for anywhere between 3-7 years. You then have to obtain your license so that you can practice in hospitals and other medical facilities.
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