There are tutorials right here at LQ....
another place often recommended here is RUTE: http://rute.2038bug.com/
When you are told to enter a command, you will do that in a terminal window--aka "console". Typically, it will be in your menus under "system" or something like that. In a terminal, you are operating in what is called the "Command-Line Interface" (CLI). This is the traditional way of interacting with computers--as opposed to the "Graphical User Interface" (GUI---pronounced gooey)
Do not hesitate to come back with more specific questions