When I took Film class a good rule of thumb is to use the 3-act idea.

 

Act 1:  Intro characters, show a bit of normal life or something mundane to show world structure.

Act 2:  Create the problem or conflict, this shows whatever they must deal with be it a relationship fall, a disaster an invasion.  Show the central thesis of conflict.

Act 3:  resolution, either solving whatever it is that must be solved or a resolution bad or good to the conflict of act 2.

 

Thats a very short hand way to say it, but in a nutshell that's how we did it in Our class.

 

on a  similar note, try story boards those help with placement, camera angles and may show you what you need that you may not know until you have to fill that camera viewfinder.

 

-- P.S.  Acts dont have to be of a specified length, some shorts have acts of differing sizes or runtimes all together but layign it our a head of time will help with your pacing, to keep it running smooth with little dead time.