Tutorials, tutorials, and more tutorials. They're free, and they teach a lot. After you get some ideas about how to make effects, start experimenting.
Sites to get you started:
psdtuts.com (perhaps a bit advanced)
photoshopstar.com
pshero.com
Also, my Photoshop life changed forever one I learned how to use the pen tool. If you don't know, I would definitely suggest googling a tutorial or two on that tool and learning.