Q: Would like to add additional screen for double screen output to my iMac 21. What do I need to get started?

I have an iMac 21.5 (late-2013) and want to add a second screen (dual screen setup). Where do I find the best screen and connectors for it to work?

Does it work from a Thunderbolt connection? If so, where can I get good information?

Using Yosemite 10.10.5 on the 2.7GHz Intel Core i5 iMac 21.