Before you start writing, you must consider the following:

1) The story arc and the plot which consists of 4 steps

Part one: Someone wants something 

Part two: Someone goes on a quest

Part three: Someone is rivalled

Part four: Someone gets what they want

2) The dynamic of all the characters

This might actually be more important than their personalities. You cannot write a story without knowing the dynamic of the characters. You can't just thrust people into a world and not know what they're all about. Ask yourself this: "Do they like each other?" "Do they hate each other?" "Will they fight?" "Are they just acquaintances?" "Will they be friends to enemies; friends to lovers; enemies to lovers?" You need to do this for every character.

Think about why these people like/hate/love each other and that might lead you to their traits and goals. 

3) The ending.

You don't have to know exactly how the story will progress chapter by chapter. You don't have to plan every detail. But you do need to know how it will end. I made the mistake of not thinking about my ending first and ended up rewriting the whole book. (You see I didn't plan the ending, so I wasted too much time before I realized that I didn't need as much chapters as I wrote. It went from being 24 chapters to just 19.) 

You must have the ending in mind before you write a single word. You have to have a destination so you know which direction you're going in. So KNOW you're ending.