You gave the answer in the question.
its like asking "where is 123 so. main st."
/ is the top level of your main partition /etc is a folder in /
typing cat /etc/X11/xorg.conf will post all the info in your xorg.conf folder
mcedit /etc/X11/xorg.conf or mousepad /etc/X11/xorg.conf will open your xorg.conf file in text editors