Photoshop's Variations and Free Alternatives
I have a different page that explains what Photoshop is, what the versions are and less expensive, or free, alternatives, here. I also talk about all the other software I use and photoshop plug-ins there.
I've been using Photoshop every day for years so its cost is negligible for what I get out of it. If I was starting from scratch and didn't have $560 for Photoshop CS or didn't get Photoshop elements for free with a scanner I'd first try iPhoto that comes for free with every Apple Mac computer, or Google's free Picasa 2 for windows, and learn those. iPhoto and maybe the others even read RAW files, so don't ignore them.
How to Learn Photoshop
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.
Basic Operation and Tricks of the Trade
All these adjustments work with every kind of image. That's why skilled photographers feel sorry for people who toil away with RAW files just so they can correct exposure and white balance later. You can do it all to any JPG image in Photoshop without all the hassle.
The tricks below presume you have at least a basic navigational familiarity with Photoshop. You can get that from any book or community college class, or even the built-in help!
Photoshop has been around since the 1980s when it was only used by professionals. It therefore uses terms from traditional professional photography and the commercial printing press trades. Often these terms are exactly the opposite of what you'd expect! The most obviously labeled tools are sometimes the worst way to do things, so read up below.
People spend entire careers learning Photoshop. There is more to learn than any single human will be able to master. Each of us learns what's relevant to our own work.