Install and use it.
Pick something you want to learn how to do on Linux (code a webpage, edit pictures, set up a home network--it really doesn't matter what) and do it. Then pick something else, and so on. Every new thing you learn how to do, even if it doesn't directly involve manipulating the OS (for example, editing a photo) will be another step along the way of learning about the OS itself.
The reference I found most useful when I started was this Garrels's Intro to Linux: http://www.tldp.org/LDP/intro-linux/html/
(It's also available in PDF and other formats on the "guides" page
I would suggest picking one of the major distros, then sticking with it for several months before you succumb to the urge to distro-hop.
Note that "Linux from Scratch" is also a book
. If you are new to Linux, don't start with that.