Which iMac model do you have? If it's an Intel iMac, you can remove the stand on the 24-inch models, but not the smaller models. You need the VESA mount adapter kit, which has the tools and instructions for removing the stand. The instructions are online.
http://manuals.info.apple.com/en/iMac24inch_VesaMountAdapterInstallGuide.pdf
Note: The adapter kit is different for the white iMac and the aluminum iMac.
If your iMac is NOT a 24-inch iMac, you can use a VESA mount without removing the stand.
http://www.imacmount.com/