A further option is to search for a furnished or non-furnished apartment by checking out the following links. You can also place an ad yourself.
In Germany, it is very common to share apartments. In shared apartments (Wohngemeinschaft, WG) several people live together in one apartment, which means that each one has his or her own room (usually unfurnished) and that the bathroom and the kitchen are used jointly. Shared apartments are mostly inhabited by young people and students. Living in a shared place is usually the least expensive option and the best way to meet people. Here are some online platforms where shared apartments are offered:
Several Berlin newspapers have large real estate sections (usually published Saturdays). You can purchase them at paper stands, train stations, supermarkets etc. or check the offers online.
Bulletin boards displaying private apartment offers can be found in the main buildings of the university, e.g. faculty buildings, library, and dining halls. Mostly young people offer their furnished apartments or their rooms in shared apartments for a limited time span (called "Zwischenmiete").