Personally I prefer learning things as they come, as opposed to trying to learn everything all at once. That gives me extra incentive to actually put what I read to practice. I learned the ins and outs of installation when I installed Slack, I learned about wireless when I set it up, and I learned how the X window system is implemented when I installed that.
To learn Linux with Ubuntu will require some extra effort, as it employs a lot of GUIs and obfuscation which make it easy to ignore the inner workings. And I would know, having spent three years on Ubuntu and not learning jack outside of the most basic commands from my first-year computer seminar. You'll have to dig a bit deeper to really learn.
You can do little things at first like force yourself to use the terminal to navigate your file system instead of the graphical file manager, or use the terminal to launch apps and install packages. Then later move on to maybe editing config files in a text editor or compiling a package from the source (even if there's already a .deb package for it!).
As far as finding these packages and apps, well you have this forum (and already some useful posts in this thread) plus just plain old Google. Get comfortable here, you'll learn more quickly!