Here are two links to show you how:
http://www.3dvinci.net/teacherguide/design_1-5.pdf
This one is a video. Showing how to make a small house for Sang, the Google SketchUp man. It shows you a different way to make the roof. You can also add a bench or other items from the Google SketchUp Warehouse. Finally, you can add shadows to your model.
http://www.youtube.com/watch?v=gsfH_cyXa1o&feature=channel