Here's a list of web sites I've used to learn how to create specific effects using Photoshop. Slittle is right, run through a few tutorials and you'll soon know Photoshop backwards.

http://www.dubtastic.com/tehmain.shtml
http://www.eyeball-design.com/fxzone/index.htm
http://user.fundy.net/morris/redirect.html
http://www.phong.com/index.php
http://robouk.mchost.com/pollb/pollinc.php
http://www.spoono.com/st/index.php
http://www.gfxartist.com/