Geman uni requirementsWatch
in order to study at a german university you have to go to a "Zeugnisanerkennungstelle" in the "Bundesland" where you want to study before starting university. It's also important to mention that you have to have your statement of results before applying.
here's an official link with more information about what you need in order for your A-level to be accepted as an equivallent to the german "Fachhochschulereife"
It basically says that you have to have AS-level German, A-level Maths and a science A-level and another A-level / AS-level if German is a whole A-Level. Note: they don't accept all subjects as a valid A-leve but you'll be fine with another science, history, english, geography or any other "facilating subject".
Hope this helps!