ehh, ok i suppose. P.S. no one is going to teach you how to shade, you kinda need to pick that up on your own buddy, people don't teach here. They send your *** to a tutorial. =P J/K, well kinda :////
1: Use the burn tool on the parts you want to be darker (outsides etc)
2: Use the dodge tool on parts you want to be lighter (Center, parts of curves etc)
3: Use the blur tool to make the shading edges blur together to look like a seemless motion from light to dark.