First you gotta learn Photoshop, all the tools, effects, and everything. You can learn this by following tutorials on the web. Once you get to a point where you feel you can recreate most things you see on the web, the rest is just creativity and an good eye.
Design everyday, whether you're tired, uninspired or whatever. Just design. Try new things, don't create the same thing over and over again, you'll get no where that way. Create a layout>get feedback>fix it up>repeat.