From a terminal as root, type:
This will bring up a series of questions you answer regarding your system (i.e. graphics card, etc.) When answering the questions about your monitor, be VERY conservative unless you are sure you know the vertical and horizontal capabilities (you could damage your monitor if you get it wrong.)
When you have concluded the dialogue, an xorg.conf file will be created and included in /etc/X11 directory. You can edit this file if you need to make any changes.
When finished, try
Hope this helps