If you are planning on obtaining a foreign citizenship, you should first apply for a German letter of retention in order to not lose German citizenship. Read the information on our German language website
Read here about the different ways of becoming a German citizen automatically.
You wish to establish the fact that you are German? Or you need to apply for a certificate confirming and proving that you are a German citizen? Read here about what a certificate of Citizenship is and how to apply.
One may lose German citizenship.
Read about the reasons for loss of citizenship and how you can avoid it.
You are not a German citizen automatically but wish to become German?
Almost always, naturalisation requires the candidate to be residing in Germany. Naturalisation from abroad is possible in only a very few exceptional cases, about which you can read here.