i'm no expert, but i use photoshop CS almost daily (photography is a hobby), and all i can tell you is read as much as you can, practice practice practice and find as many tutorials online as you can and give them a try.
I learned most of what I now know by necessity, I picked a project that I wanted to do and then taught myself PS along the way of completing the project. Just learning what the functions within PS don't really stick with me, I have to have a purpose/need and the ones that I use the most stick with me.
That being said, the main reason that i'm into it, is that my career deals with IT (computer guy) so the photography keeps the ol' gray matter somewhat balanced (but then again, after I take pictures I wind up right back in front of the computer processing all of them....
