If you are adding the drive just follow this IFIXIT guide: Installing iMac Intel 27" EMC 2429 Dual Hard Drive Kit
You will need a cable set and the tools to open your system. Here's the kit IFIXIT offers: iMac Intel 27" Mid 2011 Dual Hard Drive Kit
As for the Samsung SSD we have mostly them in our iMac's with great success.
If you are thinking of switching out your current HD for the SSD there you will need a special adapter cable to replace the needed thermal sensor which is in the HD and not in the SSD. You'll also need an adapter frame as your current drive is a 3.5" and the SSD drive is 2.5".
I strongly recommend adding the SSD drive Vs swapping out the HD.