Through force of Arms.
The Idea of a hereditary title passing from Father to Son stems from the belief of divine right. This is the belief that God has given power to that family or that they are direct descendants of God. The "first" King in the line would be up against it with people questioning his right to rule and would likely have spread this idea in order to ensure his dynasties continued power.
This brings me back to your original question, why would people give power to him in the first place?
Well after the collapse of the roman empire around 500CE a power vacuum was left in Europe. All of a sudden the law and order that the roman empire provided was gone. Looking for safety people formed groups and offered what they could in return for protection. This is the fundamentals of why the three classes of Serfs, Soldiers and Nobility were formed. The Kings or lords offered protection in return for a large portion of the food that the serfs produced.
The "first" king was probably the most respected or powerful person that had the leadership skills needed to control people, or they had divine right.
There were of course king like figures before the roman empire but it is just a good example to use because of the huge power vacuum that it left.