I would add to the suggestions of using Linux--which are on the mark--this: pick something you want to learn how to do (design a website, create graphics, edit pictures, create a home network) and learn how to do it using Linux. Then pick another one.
"Using Linux" is easier if you have a goal. Even if the goal does not directly involve using Linux (for example, many would argue that learning how to use the GIMP to edit a photo is not the same as learning Linux), everything you will learn how to do with Linux will help you a little farther along the road of learning Linux as an OS.
For me, it was the desire to self-host my website (something I no longer do, but which I did for five years). Every step I took along the road to bring it and keep it alive involved learning a little more about Linux. (Looking back, I am amazed at how much I was able to accomplish with very little knowledge and little help except my own research, LQ, and various Slackware resources on the web.)
The single reference that helped me more than any other was this one:
I still have a copy on my bookshelf.