A large portion of the work of writing a story, in any medium, is usually done before you even write the first sentence.
I’ve learned the hard way that just having a good idea for a story doesn’t mean it will magically happen. You can’t sit down and start writing that first scene, guided solely by a vague notion of Cool Stuff happening. At least I can’t. Obviously this process varies widely by author but most say they do some amount of pre-writing work before they start a new project.
I have a list of things that I need to have figured out before starting a story. Usually I draw up a cast of characters list, like the kind you see in the front of a playbill. That starts building character relationships, which will inform scenes later on. It allows me to build in a sense of symmetry to the story, so I can balance the characters motivations and let each one become a facet for the story. There is usually some sot of symmetry involved in characters. Not just hero vs villain, but a mirror image of the supporting cast as well. After all, a convincing antagonist has a support staff too. Who are they? Figuring out this before hand lets you find a n organic way to introduce those characters, rather than just have them drop out of the sky.
I also need to have some idea of the themes I want to write about. Themes provide thrust for the story. If I get stuck in a scene, I can go back to that scribbled list of thematic ideas and see which one I’m missing or have put in the wrong place.
I like to have a title, at least a working one, in mind before starting as well. Titles are to stories what naes are to people. Thy identify their boundaries, or at least give yo a sense of what those boundaries are. This isn’t necessary, but experience has proven that I won’t really have a handle on the story until it has a title. Like my list of themes, this can be a beacon when you get lost in the fog that is the day-t-day process of sitting down and banging out words on a keyboard.
I also need to know where the story is going. Not necessarily a plot outline (though I usually build one of those shortly after starting) but some sort of late 4th/ early 5th Act* moment that settles things. This ensures that I have a goal, and can steer scenes, even early ones, towards that destination. The reader may not be able to see it over the horizon but I know that’s where we’re going. And knowing where you’re going is always a good idea, in life and in writing.
* I prefer the 5 Act structure. Al ot fo writers swear by the 3 Act structure. But the 3 Act structure is crap. It forces you to stop the action and dump a bunch of exposition in Act 1, drag out Act 2 far longer than is necessary and race to have everything wrapped up in Act 3. 5 acts gives you elbow room, so that Act 3 becomes a natural climax, rather than a contrived hinge in the middle of your story.