This is a very hard system to open, more so when you have a damaged front glass & LCD.

Review this IFIXIT teardown of your system: iMac Intel 21.5" EMC 2638 Teardown.

Here is the previous model which is exactly the same as yours IFIXIT guide for taking off the display: iMac Intel 21.5" EMC 2544 Display Replacement. Here's a good YouTube vid on what you'll need to do: How To Take Apart the newer iMac 21.5"

I would strongly recommend bringing your system to an Apple Store and let them deal with the screens replacement.