http://vector.tutsplus.com/tutorials/tools-tips/quick-tip-create-a-simple-pen-tool-free-file-icon-in-illustrator/

In the following steps you will learn how to create a simple file icon in Adobe Illustrator. It’s a pretty simple tutorial that will introduce you to the basics of pixel perfect vector shape building techniques. For the starting shapes you will only need the Rectangle Tool (M), the Rounded Corners effect and some simple Pathfinder options. Next, for the coloring you will need a simple linear gradient, a bunch of flat colors and some simple blending techniques. In the final/extra step you will learn how powerful the Transform effect can be.