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Copied from GMFS PS sticky:
Here's some Photoshop links to help you guys out.
Photoshop Car/Truck Chops
The Photoshop Bible
Digimods
A Studios
Photo-Chop.com
PhotoshopJunkie
Photoshopchop.com
Those are what helped me get started but like BOTMAN said, the pen tool is what you'll use the most. Some of those links are probably dead but I'll clean up this post when I have some time, the Photoshop Bible and YouTube were probably my biggest help though