There are tons of tutorials from our community in our forum. You'll find lots of helpful commentary from other members here, too.
A complete guide to designing in Adobe, by Jellyes
A guide to coloring artwork in Photoshop, by valorandvellum
Add grungy texture to a photoshop design, by alvarejo
How to shade artwork with halftones, by ndstillie
Turn your artwork into a seamless repeating pattern, by ratkiss
Turn a scanned drawing into a design that can be colored digitally, by Mike Laughead
Dot your halftones and cross your hatches
Use this checklist to make sure your design stands out from the 1,000 or so others that go up for scoring each week. These finishing touches can make the difference between getting printed and getting lost in the Internet tubes forever.