It depends on what you want to study and at which university you want to study. Some universities don't have any additional admission requirements besides having the
German Abitur (or equivalent). Other's - usually the better ones - require a specific
numerus clausus. You'll find that information on the webpages of the various institutions.
Once you are in there, there are no tuition fees or general education courses. You might be required to pay for the student union or public transportation, though. Which is usually less than €100 per semester.