Mike150 said it all.
If I had to start from scratch, I would first do the beginner and intermediate courses on Photoshop CS5 by David McClelland available on the Lynda.com awesome website. It is great and you will get comfortable with Photoshop quite rapidly.
Then I would purchase Scott Kelby's books to further refine retouching techniques, which will make you a Photoshop expert.
Youtube is a great idea, but you need to cover the fundamentals first to understand how this software works. Otherwise you risk your learning curve to take an awkward and frustrating shape.