There is nothing much to learn about Photoshop really.

All you have to do is to play around with the tools and functions in PS and understand what each of them does.

Then, every other Photoshop effects you want can be search online using google on an on-demand basis. It's impossible to remember every Photoshop technique and effect. There are over few thousand PS techniques...

Like, today, you want to design a high-tech banner, then you search for such a tutorial. Then if you want a text effect, you then search for this again..