Yeah, GIMP is a great program (especially for the price, lol), but if you already have CS6 then I wouldn't switch. I don't use a tablet, mostly because I absolutely, positively, CANNOT draw. Wish I could, but no such. Even though I've started creating some original spacescape pieces, everything I do originates from an existing image, texture, pattern, or brush, even if I modify it to the extent it's unrecognizable.
The best advice I can think of for using Photoshop is, identify just a couple of tools that you understand how to use at least a little bit and that are useful in making your art (things like curves, or color balance, or the paintbrush, or the blur filters - or whatever you're kinda comfortable with). Then just use those for a while; get pretty good at them. Don't worry about the other 3,872 features PS "supposedly" has. Once you're comfortable with those, try out a new tool - figure out how it works and the cool things you can do with it. Rinse, repeat