I'm not a movie expert either, but for all the same reasons Bernardo suggests, I think you should create a detailed outline and storyboards rather than either a short story or a screenplay.
You want to tell the story before filming it. You need feedback to make sure the story works. However, the details you can use in a short story (where visuals are no object) are very different from those you use in filming.
For example, let's say your villian is the traditional Bug-Eyed Monster (BEM) with a whole bunch of slimy tentacles. In a short story, you can describe how the BEM slithers and squelches across the room, how the muscles undulate in each limb as it heaves its stinking bulk forward, how the acidic slime sizzles as it eats away the carpet. Scary, dramatic, effective. As a writer, you can keep piling on impossible details (the BEM can become invisible, it has basilisk eyes which turns its prey to stone, it can sing Russell Watson songs) to your heart's content.
But as a filmmaker, you have very different constraints.
You have a budget for SFX. You have to create the BEM puppet or spring for the computer and software (and tech person) to create the monster's movement. You'll need to get a greenscreen set and several suits to make the invisibility effect work. You have to get enough ear protection for everyone in the room when the creature starts singing "Faith of the Heart."
Writing an outline will help you make sure the plot works. Storyboards will help you ensure you can actually film the thing. Then you write the screenplay.