I learned almost everything I know by reverse engineering other people's designs. When you see something amazing, try making exact copies of it in Photoshop. After a number of tries, and reading tutorials, you'll be on your way.

NOTE: This does not mean rip off other peoples work and try selling it or claiming it as your own. Simply a learning strategy.

One of my favorite Photoshop resources:
http://www.tutorialized.com/