If you have a map of the city, and if you can orientate yourself - whether north, south, east or west - your problems are almost solved.
In addition, there are electronic and sketch maps at some major junctions and roads.
If you're trying to look for someone's residence and you don't have all the information - like complete address or even the person's full name - it's best to get some help from the local police station.
Having said that, people in Beijing are very friendly. Generally speaking, asking for directions from someone on the street should get you to where you need to go.