Everyone living in Germany has to register with the city they live in. So you also have to register with a registration office (Meldestelle) at a "Bürgeramt" within 14 days  upon your arrival. You will receive a registration confirmation "Meldebestätigung" which you will need for your residence permit if you are a non-EU citizen and don't have a visa for your whole stay.

Openings hours vary but on the websites of the "Bürgerämter" you can make appointments! Click on the desired Bürgeramt pin, scroll down, tick "Anmeldung einer Wohnung" and then scroll down to the bottom of the page to tick "an diesem Standort einen Termin vereinbaren".

If you are flexible concerning the Bürgeramt's location you can also book an appointment directly.