That's a BIG question. Everybody writes stories in different ways. Some start with a scene or a person or an objects and develop ideas from there. I plan my stories out. Who the main character is, what they want, and how they're going to get it. Who their enemies / obstacles are and how to fail / overcome them. I decide the start point / middle / dark moment (where all hope is lost) / and where the story ends. Then it's months of drafting the story to join up the dots. Character development and world-building are important too, so make sure you characters change (for good or bad) throughout the story and you set it in a world that feels genuine / unique / consistent. Maybe draw a map. Then, when you've finished, let some other people read it, take on their feedback and then edit, revise, edit, revise. Let more people read it and then edit it some more. And then, one day, it will be done. Good luck!
