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how, or where to start
my suggestion is to just try out various articles and tutorials, and see if you can do them. Eventually you'll amass a nice set of skills.
http://www.photoshopcafe.com/tutorials.htm http://www.planetphotoshop.com/category/tutorials/
I usually use Tutorialized. It's a pretty simple site, but it's got a few good things in there. Or if I have some specific idea in mind, I use the the best tool on the net.