This guide meant to help absolute beginners use the command line on Mac and Linux computers. For windows, you'll need to install the git command line tool or cygwin in order to use these commands. For Mac, you can open the command line with the Terminal application, inside Applications > Utilities.
This guide is supposed to be brief enough to be learned in a few minutes. For more in-depth guidance on using the command line, try this tutorial or this tutorial. Please feel free to leave comments suggesting changes,