What distro are you using? And what version?
What video card do you have? If it is ati or nvidia, have you installed the proper drivers?
If using: System > Preferences > Monitor (or similar) does not have the resolution that you want, you can try using xrandr
to add undetected resolutions:
Write back if you need more help. Provide as much info as you can. This will better enable others to help you.
And welcome to the LQ forums!