The best solution when you're just starting out is probably to think about locations you could get hold of and people you know who could act in the movie, and then base a script around that. That way it won't cost you much, and even if your actors can't act too well you can base the script around any limitations they may have.

Otherwise, for a short film you probably want to keep it short and to the point: think of the movie as though you're telling a joke to a friend, and you'll get a good idea of the kinds of things that have to go into the story, and the kind of things you can leave out. Most shorts I see start from a good idea, but are just way too long and drawn out, with way too much dialogue that's not needed.