Google SketchUp is a free 3D modelling program. It can be downloaded from http://sketchup.google.com In SketchUp you draw shapes using the drawing tools, then 'pull' them into 3D.
SketchUp is a great introduction to 3D modelling, but also has quite sophisticated tools for doing more complex tasks.
This video will show you the main features of SketchUp and how to get started using it:
Click View > Toolbars > Large Toolset to get a much bigger set of tools.
Middle Click to orbit. Use the scroll wheel to zoom.
Leave the pan button selected, then left click with the mouse to pan.
Space bar will switch quickly to the pointer tool.
What Do All the Tools Do?
A good way to become familiar with the SketchUp tools is to use the built in instructor. Click on Window > Instructor and a small window will pop up. Now as you click on any of the tools in the toolbar, the instructor will show you what it does and how to use it.