iMac Late 2015 thunderbolt cable to connect to Monitor with Display Port

I'm sorry if this question has been asked before, but I cannot find the answer.

I have a Late 2015 27 inch iMac with 5K Retina Display. I would like to connect a compatible 2nd monitor. I bought a Samsung, but the display is horribly fuzzy. It may be the monitor is not suitable or it may be the connection I am using. I am using a thunderbolt to HDMI cable. I think I need to change this to either a thunderbolt to Display Port (the samsung has a display port connection -- not mini) I do not believe I have thunderbolt 2, I think it is just thunderbolt 1. Should I get a Mini-Display Port to Display Port Cable? There are too many options and I'm confused. Is it worth it to try a different monitor with a 4k or 5k display? I'm just using it for standard work...not video or graphics editing or gaming, but I would like to a display as close to the resolution on the iMac as possible/affordable.

Thank you for any assistance that can be offered.

Kathy