What you should really attempt to do in mastering a medium is to be able to state your planes and values correctly. Then learn to make sharp or soft edges. Learn to achieve a soft gradation of tone when wanted, or a crisp and blocky or sculptural effect. Learn to place accents of light and dark. That is all there is in any medium, but it is not easy as it sounds. (Loomis, Andrew. Creative Illustration. Viking Press, 1950.)
Improve your Art:
Hey guys, if you're looking to improve your art in different ways such as: learning how to make comics, color theory, and composition try Skillshare.com. If you send me a PM with your e-mail I can send you 3-months for $0.99.
I love you guys and want to see your art improve and I also want that free month subscription for each member that I refer so yeah, check it out.
If you're looking for books on ways to improve, try Sycra.net under resources. Your local library should have a lot of those books because they're so well known.
To learn the ins and outs of Photoshop, I'd recommend CtrlPaint.com. It has a free library with like 100 short videos on how to use Adobe Photoshop.
Tip: If you use videos from Skillshare or sites like CtrlPaint.com, keep a note book with the most important details because it is easy to forget. Some of the videos on Skillshare.com are over 2hrs long but they're broken up into smaller videos that make it easy to learn a little at a time so stop after 45mins and come back tomorrow or take breaks and finish it at your own pace.
I would just read the books from Sycra.net without taking notes though. You can always refer back to an important page or photocopy something that really stands out as complicated and tack it to a wall in your room/studio.
I hope that helps.