How to set up dual monitor for mac?

i have imac retina 5k 27 inchi, late 2015 and i also have dell monitor which support VGA, HDMI, DVI input. i want to use dual monitor for mac by using this dell monitor as the second monitor, my question is how can i do it? should i use cable converter to connect it? is there setting should i change in mac, if yes. what is that? and how to setting it?c

here is the specs of imac:
iMac (Retina 5K, 27-inch, Late 2015)
Processo: r 3.2 GHz Interl Core i5

Memory: 8 GB 1867 MHz DDR3
Startup: Disk Macintosh HD
Graphics: AMD Radeon R9 M380 2048 MB