Berlin is not a city to be travelled on foot – too spread out. I picked up a (probably stolen) bike at the Flohmarkt (flea market) in Friedrichshain for €35.
Berlin is, however, a city made for biking. Literally, there are bike lanes everywhere. Most streets have a painted line for the bike land or a section of the sidewalk marked for biking. Most roads also have a short marked lane at intersections to give bikes room to skooch up to the front without getting squished.
I absolutely love biking in Berlin!