After you know the principles of Photoshop I would HIGHLY! recommend these video tutorials, I got them for Christmas and was in a very similar situation to yourself myself. I now have the confidence to go out and texture anything, sort of...
:: Eat 3D :: Old Pillar
:: Eat 3D :: Next-Gen Texturing
Another resources I would highly recommend is the articles on this website, they are infinitely rewarding and just a few hours of reading could teach you the theory and principles behind next generation texturing.