The most comprehensive and explanatory would be lynda.com but that is a pretty expensive resource if your are not working with a team of creative artists. Otherwise you can see a lot of tutorials on youtube, blogs, deviant art, and good-tutorials.com.
Play around with the program. That's the best way to learn.
A few pointers. 300 square pixels per inch is the resolution you want to do your photo and design work at. CMYK color mode is for print designs. RGB color mode is for photography, and screen displays. Learn Curves, Levels, Color Balance, and Blending modes first. As well as layers and masking. That should be the most important stuff.