The best way of learning how to use photoshop:
PLAY :)
You don't need a manual to learn it, it's very much simply playing around with the tools. Using software to create art is hardly different from using real media. I basically paint on PS as if I were using acrylics xD (It's kind of easier to use PS to paint anyway).
Oh and here's a tip for that sketch. Your scanner (or camera?) is pretty cruddy. A good way of imporving the quality of the file is to play around with the curves.
Go to image >> Adjustments >> Curves or simply hit ctrl + M
Then you play with it, I'm not sure how it works, so just drag that wiggly line around until you think it looks nice xD
Your sketches are pretty awesome, but I think you need a bit more practice with them lighting and shading junk. It makes you look awesome and have more bragging rights xD
ONLY REAL MEN WEAR PINK >O