Your 2011 iMac will support the "VESA Mount Adapter Kit" which will allow you to remove the iMac's stand then once installed will allow you to use any mount or reticulating arm or other wall mounts that support the VESA standard.
These Google searches would be useful.
- iMac 2011 VESA
- VESA Mount Adapter Kit
You also may want to confirm that your iMac is indeed from 2011 as later models do not support the removal of the stand.