If you don't know how to coloring, please click this link : Basic Coloring.
This tutorial is really useful to practice shading and shadowing on your picture.
Now, let's start with this simple colored image :
Choose a light source, anywhere you like, I will describe the detail you must remember below :
OK, Let's pick Dodge Tool
(make pic lighten) and Burn Tool
(make pic darken)
Follow the rule on the upper pic and start shading!
First, pick Burn Tool
, choose big size brush, about 60px
Shading on the rear of body :
Continue shading on areas where aren't lighted :
OK. Let's pick Dodge Tool
, option :
Remember follow the rule!
Shading on areas where are lighted....
OK. Pick Elliptical Marquee Tool
Change foreground color to Black.
Create a new layer and put it under others.
Draw a small ellipse at bottom of subject :
Press ALT+BACKSPACE at the Same time to fill the selection with Black.
Cute? ^.^
The end!
More Tutorials: 
