This is a very important question and one that needs to be told to the world. This regime is one of the most evil in the world and they rule the country in a few ways.
First, they rule by propaganda. There is a true cult of personality. They believe that their rulers are divine. They also state that the world is a better and safer place because of the Kim family. You might think this is outrageous, but when the country is cut off from the world, you can easily be brainwashed.
Second, there is also sheer intimidation. If anyone questions the government, they are put into labor camps. Of course, North Korea denies this, but according to satellite images, it is clear that these labor camps exists. In these camps, people are tortured, killed, and worked to death.
Third, North Korea is also a very poor country. So, they also are able to rule, because the one agency that has power is the government. In other words, the people have no options.
For a good book on this topic, read, Escape from Camp 14 by Blaine Harden. According to George W. Bush, this book was one of the most important books he read during this presidency.