There are a large number of websites that will help you find a room in a flatshare (a “WG-Zimmer”) or a whole flat for yourself. The most popular are:
Many Berliners travelling for an extended period sublet their rooms or flats. These sublets, called “Untermiete” or “Zwischenmiete,” generally offer the advantage of already being furnished. Insist in all cases on a subletting contract (“Untermietvertrag”) – do not take over as the principle leaseholder!