I'd recommend watching this tutorial:http://fxhome.com/support/video-tutorials/view/16/keying
It explains the basic keying techniques. If you're not yet familiar with the interface, you should have a watch of the 'Interface' tutorials too.
Once you understand the interface, the basic process is as follows:
1. Import your foreground and background to the media browser.
2. Put your background on the first layer of the timeline.
3. Put the greenscreen clip on the layer above the background.
4. Go to the Matte->Key toolset for the greenscreen clip and add an 'Auto colour difference' key.
If you change the canvas view to 'project overview' you should see it composited.