Command Line Interface (CLI)

Understanding the command line is crucial to getting started.

Keyboards & Command Line Interfaces: Crash Course Computer Science #22 - a great way to introduce the command line.
How to use the command line on Mac - a good quick tutorial, he goes through this guide.
Windows Command Line Tutorial - 1 - Introduction to the Command Prompt - a good intro for people using Windows. 1 to 5 is necessary.
Learn Enough Command Line to Be Dangerous - an indepth way to learn the Unix command line interface (CLI).