A university entrance qualification (usually the Abitur) is the basic requirement for studying. But even without an Abitur, studying is not impossible. Please refer to the information on the following websites:
For international prospective students there are other requirements for studying. They must also complete the online application for non restricted subjects.
Some programmes require proof of certain skills and knowledge, sometimes even before you start your studies. For example, to study sports science you must pass a sports qualification test, while other subjects require knowledge of certain foreign languages.