This is a tutorial for Adobe Photoshop, but can also be used for Gimp or any other program with layers, transparency and a dodge/burn tool.
This is not at all about drawing an eye, just showing a way to color it. If you want to know how to draw an eye I suggest you to read Danas Eye Tutorial instead :)
I will assume that you have basic knowledge of Photoshop. You may ask if you don't know where to find a certain function that I have described here, of course.:)