Good Morning,
I learned Linux back in college on a PowerMac 7100 running mklinux. I found a couple books very helpful along the way, and from there, got more involved with the OS to the point that I obtained a server-class machine with a RAID system to just keep all my stuff together.
You will find Linux documentation all over the place -- on the web, in a book, in a discussion you have with friends.
Think about what you want to do with the OS. There is no such thing as "learning everything". For example, I like to write out my DNS files, so I have never learned to use the simple tool that RedHat ships along.
My suggestions:
1) Pick a Linux flavor and run with it. There are many to choose from: RedHat (now called Fedora), Slackware, Mandrake, Debian, Knoppix, YellowDog. I would have to say that RedHat is the most popular, but they also did something to really shift things around by starting the Fedora package, and they recently went politically correct. I did not like my experience with Debian. Some people might lump FreeBSD into this category. But pick one and dig deep into it.
2) Run the OS often. DO things. Setup DNS. Setup a print server. Setup Samba. Setup netatalk. Setup mars-nwe. Setup NFS. Setup VNC and remote control things. Setup IPTABLES and do firewalling. It is easier to learn when you define the problem to learn from. No one goes out and just learns something. I learn how to bake a chocolate cake because I want to eat some. Same with computers. You want/need DNS? Then learn how to do it.
3) Think of all the things you do in Windoze. Try to do them on the Linux box.
4) Try programming. C++ or Bash scripting. Expect scripting. Try to automate things on your Linux box.
5) If you have gotten this far without making a normal user account, consider yourself lucky. Make a test user and use file permissions to seriously limit functionality and access to your box. Do things still work?
And most importantly, give yourself time to do these things. Forget the cert books right now. You are in a pure learning mode. Go out and have fun with it.
Christian