Do you decide what kind of law you want to practice when you're in law school, or can you decide after?

What I'm trying to find out is, can anyone become any kind of lawyer they want after they get their JD, or do you have to take courses specializing in a specific field, i.e maritime law. Do law schools mandate that you must take classes dealing with the kind of lawyer you want to become? I just find it hard to believe that with a JD you can become both a small crappy law firm and also a lawyer that deals with human rights/environmental issues...Can anyone answer this question?