Birth control help

Hi, I'm 17 and I'm going to the gyno for the first time on May 16th and I'm planning on getting birth control because my cramps when I'm on my period are so bad I'm brought to tears and almost puking. But I am trying to do research on the Pill and I keep seeing there are different kinds of pills, what are they and what do they do and do all of them prevent pregnancy and how do I know which one to get