Birth control?? EC??

After reading that.. would going on birth control do anything now?? I don’t really know anything much about it other than the obvious.. it prevents pregnancy.. if that would be useless.. do I have do have a prescription or anything to go to a pharmacy and pick up a plan b or something?? Also are there any dangers of taking it?? Do I even need one?? I pretty certain I’m fine but I keep thinking what if?? Help😅