I'm currently writing a book and can give insight on my own experience.
If it includes characters, start with an introduction and tell briefly about them. Show their character traits when the story progresses into the middle of the story. Let the conflict build up to move onto the main part of the story.
If it's an essay, briefly explain your topic. Give short reasons why in one sentence. Then move onto the body paragraphs to explain one of your three reasons in each. If you have sources use them.
I hoped this helped you.