Every society has at least one leader. How that leader came to power is determined through the type of government that exists. For example, in a matriarchy women are most powerful within the government and within families. A geriatric society holds elders in high esteem. Many fantasy stories feature cruel dictators who torture and imprison enemies in order to maintain tight control.
One way to decide what type of government your world will have is by doing research into various forms that have existed throughout history. Model yours after the one that you feel most comfortable writing about.
Remember that there can also be shadow groups that are trying to influence or seize power. You can have secret societies, militias, strong religious leaders and guilds or unions that control decisions made.
Your task is, once you’ve chosen the form of government, to write a scene in which conflict arises because of a challenge to the throne or seat of power. The purpose of the scene is to propel the story forward, to provide tension through obstacles that your main character has to overcome.
Have fun with this one.