A fun place to start is with simple coloring book coloring to learn some of the tips and tricks. I just posted a thread in the Art forum called "Coloring Book Fun - Fat Cat. It's a place to start.
Fat Cat
A hint... make a new layer and set it to multiply mode. Grab your paint brush and start having fun. You might want to make new layers for color groups (blue, green, red etc,) so you can go back and change things.