Thunderbolt to HDMI connection

I have a 2013 iMac with a Thunderbolt Port. I also have a GA-Z77XUD5H, i73770, 16GB Corsair 16600Mhz DDR3, GeForce GT640 and Mac OS 10.8. The GeForce GT640 has an HDMI Port. How can I use my 2013 iMac’s Thunderbolt Port as a monitor with the GeForce GT640’s HDMI? I live in Canada and can be emailed at thedogtoy@gmail.com. Thank you for all responses.

Angus MacKinnon