Google Sketchup is a great product. If you want to know how to use it,here is a step to step manual about using this software. Begin by downloading a free copy of the software. It is available from http://sketchup.google.com/download.html.
A polygon is "traditionally a plane figure that is bounded by a closed path or circuit, composed of a finite sequence of straight line segments". This article will show you how to make a polygon building using SketchUp.