Video 163
This video gets you started with displaying buildings in 3D in QGIS. It uses the Extrusion option on 3D View of the layer properties.
Ideally, you need a elevation model (DEM) to properly display your features but you can effectively snap your 3D features to the Online terrain provided (as shown in the video).
Another key thing demonstrated in this video is the new linking of 2D and 3D windows so any map extent change in a window is duplicated in the other window.