To get a room or apartment in a student residence, you need to contact the student services organization (Studierendenwerk) at your university. Check their website: in addition to information about student residences, conditions and deadlines, you will find the option to apply online. You can also ask the International Office at your university for help with your search.
If you are looking online for a room in a private shared apartment (WG) or a small apartment, platforms such as WG-Gesucht or Kleinanzeigen are a good place to start. With a bit of luck, you will also find offers in your university's social media groups.
If you haven't found a room by the start of the semester, you will need temporary accommodation. Look for hostels, youth hostels or guesthouses nearby until you can sign a permanent rental agreement.
Tip
Housing is scarce in metropolitan areas and popular university towns. To increase your chances of finding an affordable room, you should start your search six months before the semester begins.