A huge advantage of Photoshop is you have more ways to learn it, like this page here, than any other program. So many people know it it's easy to learn just about anywhere.

The best way to learn Photoshop is to take a class at your local community college to learn the basics of Lightening Underexposed Images and Correcting Color Casts and White Balance Problems. These basic adjustments are covered in every introductory Photoshop class and book so I'll just cover them quickly below. You must be fluent in these to get anywhere with Photoshop for optimizing images.

Photoshop also has built-in help as well as free tutorial usually included in the box. All you need to do is choose how you learn best and go that way. Personally I learn best from an in-person class or having an expert show me in person. When I have a question about how to find some obscure tool or how to make a command work I just choose help from the HELP menu.