It might be easier for us to help you if you could tell us which model of monitor and tower you have, and are you actually intending to mount the monitor onto the tower case? if so, why? if not, what are you asking about?
Plug in the VGA cable from the monitor to the video card on the comuter. Monitor should come with the cable, or you can use the old cable if it's removable from the old monitor. Then match the screen resolution in Windows to the monitor's resolution.