I dont understand how the military works??

Lets say I want to join the Airforce or Army to be a pharmacist or iono gynecologist. I know they just dont give u your education but u gotta do something in return for them? but what is it? going to boot camp or serving them as a gynecologist or pharmacist?? Im interested only cuz theyll pay for my education cuz med school is reaaally expensive and as far as i ask people who are enrolled they wont tell me what it is u gotta do for them.....