In principle, German citizenship is the prerequisite for becoming a police officer in Germany. Police forces are categorised as civil servants, and, according to the German law, one may only work as a public servant as a German citizen.
In some federal state, however, , there are laws according to which one can become a public servant even without a German passport. Therefore, under certain conditions, you may be able to start police training, even if you do not have German citizenship. To find out more about the relevant rules and regulations in your federal state, get in touch with the Police Union, your local police or www.polizeitest.de. There you can also find out about the other requirements, such as school leaving certificate, physical fitness requirements, etc.