Install a Linux Distro inside a Virtual Machine:
http://www.psychocats.net/ubuntu/virtualbox
http://www.virtualbox.org/manual/UserManual.html

Here are instructions on how to make a virtual machine using Virtualbox, and
how to use it. These instructions are for windows, but it is basically the same.


IBM Professional Certification Study Guide:
http://www.ibm.com/developerworks/linux/lpi/

This is mostly/all CLI stuff, and talks about the different portions of a Linux OS as you encounter them.
This is what professionals do to learn about the basic structure of the operating system, as well
as some cooler things you can do with it. Great Reference

Linux Directory Structure:
http://www.tuxfiles.org/linuxhelp/linuxdir.html

What do all these system folders do? Look here to find out...


WAT DO I DO I AM NO GOOD WITH COMPUTER:
http://www.justlinux.com/nhf/Installation/Familiarizing_Yourself_With_Linux.html

If you find yourself in a shell and don't know what to do or how to do it, look here for the answer.


Cheat Sheets:

Navigating In a Terminal:
http://fosswire.com/post/2007/08/unixlinux-command-cheat-sheet/

Basic commands for moving around your system from a terminal.

The VI Text Editor:
http://www.math.ntu.edu.tw/~wwang/mtxcomp2010/cssd2011/download/vi_cheat%20sheet.pdf

vi is one of the most popular text editors ever made ever. It is, however
complicated to learn to an asinine extent. Since old computers had very limited
networking capabilities, VI aimed to cram as much functionality into as little space
as possible, which translates to short, indecipherable commands. Do not expect to
master VI anytime soon. When you do get proficient at using VI, you become unstoppable.
Take the time to learn it if you ever want to do what I do and make ridiculous monies