I learned Photoshop merely by messing around with it on my own for the past 4 years or so ... however, I don't suggest that at all ... if you need any specific help or general starting hints, you can ask me, or I'd recommend Smack, he is epic at Photoshop ...

Layers are, in my opinion, the most important part of Photoshop, because of how useful they can be when used right ... learn layers, and you'll be ready to take over the world ...