The usual method for entering text commands is to start a terminal. Several terminals are started when Linux boots. You can access these by holding down <control> and <alt> and keying any of the function keys from 1 to 6. Holding down <control> and <alt> and keying F7 will get you back to the graphic screen if X-windows is running.
You will quickly notice that you have to login to each of these terminals even though you are logged in somewhere else. This is because Linux is a multiuser environment. Each of these terminals creates it's own user and task space before launching it's own command shell.