The file you want to edit is called xorg.conf it's location is /etc/X11/xorg.conf
here is the two parts you will want to edit.
Identifier "Generic Monitor"
Use a editor to edit it nano, vi, gedit, open a terminal window and login as root the first thing you want to do is make a copy of your xorg.conf file.
Change into the X11 directory like so cd /etc/X11
make a backup of xorg.conf like so cp xorg.conf xorg.conf.old
Now edit the file editor-name xorg.conf
put in the right Horiz and VertR for your monitor.
then edit the modes section put the default you want to have at bootup to the left, like on this one default would be 1024x768 then when your done save and exit then restart Xserver
You can restart X by pressing Ctrl+Alt+Backspace at the same time.
Modes "1024x768" "800x600" "640x480"