What is the problem with the way it is currently displaying the image?
Is the image distorted?
Are the edges cut off?
Are the colors off?
Are you sure you set it to the correct resolution?
Usually, you can set the height (vertical size) and width (horizontal size) of the image on the monitor itself. Some monitors have an auto adjust which will automatically correct the image to fill the screen. As for setting the resolution, that needs to be done on the PC. You would need to change the resolution if the image looks distorted.
What video card are you using?
What operating system?
If it is Windows, you simply go to the desktop, right click anywhere and click "screen resolution" and make the adjustments from there.