Just start pushing buttons... Experience is the best teacher. I first started using Photoshop back with version 4.0 and have never taken any classes for it. I now use version 8.0 (Adobe CS). It's one of my favorite programs ever! Very, very powerful tool.
Life would not be complete without a photoshop contest. The image at the top is the original and the following images are shopped. Some of them are great. Disclaimer: NSFW means Not Safe For Work. Don't go there. (At least they warn you, all others are mostly rated G)