First get out of X windows so you can make changes. Change to runlevel 2 by typing 'telinit 2' in a console windows (konsole/gnome-terminal/xterm/aterm etc).
Then use an editor with which you feel comfortable to edit the file I mentioned above. You'll only have one of the 2 so which to edit isn't an issue. As a beginner I would use something like Pico. The ^ symbols on the menu at the bottom mean to hold the control (CTRL) key. e.g. CTRL + o saves a file to disk.
create a backup of the file first. e.g.:
cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak
then if it all goes wrong, do:
cp /etc/X11/xorg.conf.bak /etc/X11/xorg.conf
to get your original configuration back.
All this as root of course.
To try out your new config type startx
(you may know this, but I don't know if Debian is text mode or graphical mode login by default).
If you want to get back to graphical login then type: telinit 4
(or it might be 5 in debian, I don't know)