In principle, you have to hand in a copy of the following documents:
- Passport (or ID card) and residence permit
- Marriage certificate if you are married
- Birth certificate if you have children
- Salary statements which prove how much you earn per month or a certificate from the Jobcentre or the Social Welfare Office
- Confirmation of Registration ("polizeiliche Anmeldung")
- Bank statements
The required documents may vary in each state. Therefore, it is best to refer to the application form provided by the Housing Office responsible for you. There you can find a full list of necessary documents you need to submit. You can find the Housing Office responsible by googling the word "Wohnungsamt" and the name of your district.
Please note: If you are uncertain about the required documents, do not worry! You can still submit the application. If anything is missing, the Housing Office will send you a letter with a list of documents you have missed.