There are a bunch of great tutorials that come with photoshop itself. I learned how to use photoshop by using the tutorials inside photoshop as well as using free photoshop tutorials on the internet. Here's a link to absolute cross:
Photoshop Tutorials -- Absolute Cross
Also, check out phong:
phong » tutorials [ photoshop, flash, pov-ray & polar graphing tutorials ] - His tutorials make some really neat effects.
Now for the most part, the tutorials on the internet aren't good for prepress but do get you familiar with the tools. In my book, being familiar with the tools is what photoshop is really about.
