It depends on what you want to write.
First, start with the setting. Choose what you want to use, the era used, the level of technology, sci-fi or fantasy or real world, even the religions. Now, depending on the genre of your story, a few things will change. For example, if you go for an action story in a fantasy or sci-fi setting, you have to come up with a power system. Is it a locked power system or a growing one?
Once the settings are set, you can start with your story. The first point will be your MC. Ask yourself what you want him/her to be in your world at the beginning and end of your story.
Now, you just have to write what comes to your mind for the story. Try a lot of things, don't mind just writing anything. You can change it when you want if you didn't upload it right away. Once you find the way you want to write your story, continue with it.
If that can help, you can write a story planning. You don't need to write all the details of your arc and future arc, just some points that you want to write about that will help you to lead from one arc to another smoothly (or at least it will help a bit)