If you didn't order the iMac, presuming that's what you mean, from Apple with the VESA mount included, then you can't install a VESA mount now since the stand is not removable. This is the only solution I'm aware of:
http://imacmount.com/order.html
If you are actually referrring to an external display of some sort, you'll need to tell us the brand and model of display.
Regards.