The Einwohnermeldeamt is the local government office for registration of residents. In Germany, the address of every resident is recorded through these offices. If you are staying in Germany for more than three months, you must go to the office in the district where you are living to register, within 14 days of your arrival. This office also issues personal tax cards, for those who require them.
Forms are available at the institute.
There are different offices in Berlin, depending on the district in which you live.