For late models 2012 and so on you have to order it with the VESA mount when you buy it. Otherwise you will need to either disassemble the iMac to get the base off and find some adapters that work or get a mount that handles the base (there are some out their in the market).
I'm looking for a wall mount with some mobility in many degrees, and I'm still looking.
Do you want to mount it on the desktop or to a wall?
You cannot remove the stand on the retina like you used to be able to on the older Macs.
Your best bet is to get it with the vesa mount option from the Apple store. The only point of note is it can make selling it a little more difficult if you plan to in the future.