Practice pencil sketching! painting is alot harder than pencil sketches. Start out w/ black n' white, then move on to color. Thats how i got decent at drawing. (Hope i made my 1000th post count.)
Draw slowly and color everthing one at a time. Don't be afraid to go over some lines when you're coloring, you can always trace them in the end. Save after every step in case you mess up, when your done delete all the paintings you don't need.
@GGAnth Not too sure where you're coming from on that; even with a drawing tablet or entire tablet PC, the large screen size would diminish from the level of detail. With a mouse, it becomes more difficult.