You have a good start here.
Draw from life. Start the figure with the basic shapes. Get a good grasp on human anatomy. Don't look at how Jim Lee draws the muscles, look at the real thing.
Even if you want a cartoon style you still need to learn how to draw the figure properly with correct proportions and understand why you're drawing the lines you do.
"Life Drawing" and "Constructive Anatomy" by George B Bridgman are good books as well as magazines showing figures....muscle, sports (basketball is the best) to help you along the way.
How to Draw Comics the Marvel Way won't hurt either.
Go Here www.scottmcdaniel.net and check out the Drawing Comics section. Go through it all.