Welcome to LQ. You should provide more detail?
What video card? What monitor? What commands did you try? etc...
And, like korozion mentions, xorg.conf, in some distros, is the file to look into. In Red Hat 9 I believe the file you need is /etc/X11/XF86Config (might be XF86Config-4).
And probably you need to edit the monitor section with the horizontal and vertical settings for your specific monitor as well as the depth and resolution mode lines, like these in my ubuntu system:
Identifier "Generic Monitor"
# Option "DPMS"
Identifier "Default Screen"
Device "NVIDIA Corporation NV25 [GeForce4 Ti 4400]"
Monitor "Generic Monitor"
Modes "1280x1024" "1024x768" "800x600" "640x480"
Also, click on the Search button at the top of the page. This question has been asked many, many times.