Check this out: http://modthesims.info/ (If you're not into Sims, that's fine, it's just an example.) This is all stuff done by people who just decided they wanted to change how something in the game looked, or how their Sims behaved, or they wanted to add an item that didn't exist before, or to convert an object from some other game. So they taught themselves and each other how to make and manipulate 3D models, textures and bits of code to get the results they wanted. Some of them even sell the content they create.
Is there a game you like that you've always wished contained a particular item or option that isn't there? Google and see whether there's a modding community for that game. You may be able to learn how to add it yourself. That's the best motivator I know of for learning these things, is to think of something you'd like to actually have and use in your own game, and then learn hands-on how to make it happen.
Don't expect it to be instant or easy, though. It does take work. Anything worth learning does.