Given the sample context I would say:
Before the exam you have to prepare for it.
During the exam you have to be prepared in order to get a good score.
I have to prepare for the exam because in order to get a good score I have to be prepared.
So, "have to prepare" is an action that happens before the time you need to do a test or an exam. You have to prepare for the soccer game otherwise you aren't going to play well.
While "have to be prepared" is something that happens after some preparation in order to avoid problems in case something goes wrong. Example: you have to be prepared if you want to leave your parents house, because you'll need a way to pay for your rent.