After you arrive and get rested, the first thing you need to do is to do the city registration or Anmeldung. This should be done within 2 weeks of your arrival. You get two documents, one document stating that you are residing at the address that you provide and you receive your tax-id by post. Most cities have their own website and you can check it out, some cities also made this process online. The state of Baden Württemberg is one of them (refer to the link below). It is a simple form and one has to submit scanned copies of passport and rental contract.
If this does not work out, you have to visit the local Bürgeramt (Citizens office) with an appointment!. I had submitted my documents online but I did not hear from them for a couple of days so I went to the Bürgeramt unannounced but they sent me back. They stated that I should book an appointment and come back. Luckily, I did not have to go through that hustle as the next day, I received a email from them stating that my city registration is ready.
In Heidelberg, you can apply through the Service portal website mentioned above and can also check the city’s website (link below) for more details. The website also offers multiple online services and details of local Bürgeramts.
If you move to a different locality or even the same building, you have to deregister an re-register with your new address at the Bürgeramt.
