Read screenplays for short and full-length films, which you can find in bookstores or online, to learn formatting. Slug lines, action, transitions and dialogue need to be placed in the proper location on the page. This helps the other members of the crew when filming. For example, the cinematographer needs to know whether the shot will be indoors or during the night for lighting purposes. A slug line might read, "INT. JOE'S KITCHEN -- NIGHT."
Come up with an idea with a beginning, middle and end. The biggest hurdle for short films is budget, but short films can be about anything. They should tell a story though. A man waiting for a bus isn't a very dynamic situation, unless the man is waiting for a bus to take him to the hospital for the birth of his child or if he's waiting for a bus so he can jump in front of it. The idea should take the character on a journey and force him to overcome obstacles.
Create a list of characters. Depending on your budget and length of the film, it's safer to limit the number of characters. Your protagonist needs to have a goal, something she needs to achieve by the end of the film. Primal needs, like saving a child or winning the heart of a lover, raise the stakes and keep the character motivated to overcome obstacles.
Outline the film, listing the opening image, the setup, the inciting incident that throws the story into motion, the journey and the conclusion. Short films typically work best when the story gets set into motion very quickly. For example, if your story centers around a robbery, the gunman or thugs should appear on screen in the first few pages, possibly even on the first. It's all about compression.
Write the script following your outline. A page of script typically translates to 1 minute of screen time, and while short films can vary from 30 seconds to 40 minutes, shorter films have a better shot of making it into festivals, because they take up less time during the screenings. There's no rule about how long a short film should be, but the key is to tell the story in the most economical fashion. Don't include three locations when one will suffice.