I'm guessing that you want to enter commands in a terminal---AKA the "command-line interface" (CLI).
In your menus, you will find something called "terminal". Open this, and type your command. Depending on what version of Linux you have, you will probably be using the BASH language. There are MANY tutorials on this, but I recommend the BASH Guide for Beginners. You can get this at http://tldp.org