I took a course in it. Look into a local College class.
But even in taking that, you need to play around with it a lot to learn it.
Also, subscribe to get the Adobe Mag. There is a lot of info in it.
It takes technique. Photoshop is like a mechanics tool box. Every tool has a different use, yet just because you know what the tool is for doesn't mean you know the best way to use it. It takes practice, and lots of it.
Photoshop can do anything. So much so, that pictures don't hold their value as evidence so much in the courts.
You also have to have a love of it, and using it.