As root: in /etc/inittab you have to change some value from 5 to 3, be EXTREMELY careful. (make a backup of it!)
su root #makes you root until you type exit (or ctrl+d)
ctrl+alt+F1-6 puts away GUI, ctrl+alt+F7 to get it back
Also you should try to find some site with the most common cmd's (it got tons, but they are in Norwegian) You can learn a lot from them.
You should learn using linux without GUI. When you get used to it, it's a lot faster. (I'm talking about terminals in the GUI now)
You should try and feel out linux a little more. I remember wondering about how to turn ON the GUI on startup for 2 weeks ago. Google told me in less than 5 minutes