In Germany you don't have a grad-school system like in the US. If you want to earn a Ph.D degree, you must apply for a Ph.D research position at a university. So, you must be qualified to do research, although some training will be offered throughout your appointment to you, of course.
There is no tuition fee, you are a full time employee of the university or some externally funded employee at the university. Usually you'll be required to have a masters degree, but it's up to the Prof. you want to be working under, to judge whether you are qualified or not.
So, you should make sure you are pretty much ready to start to do the research you want to do for your Ph.D. It helps if your Masters thesis is about the same subject, and if you have already published peer reviewed articles on the subject. This is not absolutely necessary, but you can forget about the Ph.D. position if some other candidate does have these qualifications (which is not very unusual in our system).