Locate and press the "Auto" button - next to the "Power" button. The monitor will automatically self adjust to the analog signal (a standard 15 pin VGA cable).
If you are using DVI (Digital Video Interface cable), you'll have to right click on the desktop and change the resolution, scan frequency or other setting - depending on the OS (Operation System) such as WinXP, Win 7, Linux, Mac, etc.. Each OS has a different way of getting to those settings.
You can download a copy of your monitor owner's manual
here. Please rate this reply - good luck!