To register as a resident in Germany,
Apartment and WG contract (Mietevertrag)
Confirmation of residence of the landlord and WG lender (Wohnungsgeberbestätigung) The format of Hamburg is here
Go to Bezerkamt under the jurisdiction of the residential district and tell Ich würde heute anmelden.
Especially note that residence confirmation is required from 2015 so be careful so as not to forget it.