There are basically two ways to go about this. One is the old-school way, like sitting through a formal class and grinding through lessons in order. The other is just learning on the fly—you look up how to fix something specifically when you actually need to do it. That’s how I handle Adobe Photoshop. It’s way too massive to try and memorize everything, and honestly, you never really need to know every single feature in a program anyway...
For instance, let's say you have a photo that just needs a quick touch-up before posting it online or printing it. In most cases, that's all you'll ever need: maybe a quick crop, some color correction, adjusting hue and saturation, or messing with light via contrast, levels, and curves. You might need to fix red-eye (which you can do with masks, though most software has wizards for it—better to learn how to do it manually though), plus a little retouching to clear up a pimple or remove some clutter from the frame. Then there's sharpening details or blurring out the background...
Just getting a handle on those basics takes some time, especially the red-eye stuff and retouching...
And before all that, it wouldn't hurt to at least understand the fundamentals of photography, color theory, composition, and stuff like that...
Then you get into heavy photo manipulation. That's where you start dealing with layers, selections, masking, transformations, and all sorts of filters to make things look realistic. Every one of those topics is its own beast. It's best to tackle them one by one depending on what you're trying to pull off. Honestly, the best move is to just grab a camera, mess around with whatever's in front of you, and play with the shots. Like, if you edit a photo of a friend to slim her waist, boost her chest, and wipe away her dark circles, she might buy you a beer... or start a fight with you if you go too far...
Don't freak out just yet, but yeah, there's a ton to learn. I've been using Adobe Photoshop since '95, and I'd say I probably use maybe 40% of what it can do. And really, I don't even need more than 20% for anything I actually do...