Write an outline. Once you have an interesting idea in your head, it's time to flesh it out. An outline does not have to be terribly detailed, but should give a sequence of events that happen in the story and when. Every story is divided into three acts. This is just a rough guide for you to follow. Your outline should be more detailed.
Act I should be fairly short and to the point. Act II should have the heft of the story. Act III should be short and succinct.
Act I
A. Here you'll introduce characters and settings.
1. Present a problem or the protagonist's goal
B. Here your character(s) will make a plan.
C. Characters start to attempt to work toward their objectives.
Act II
A. Characters are presented with other obstacles as they attempt to achieve their goal. The plot should pick up.
B. There are failed attempts and more obstacles.
C. Work toward objective and encounter more obstacles; rising action.
Act III
A. Here is where the story wraps up. You'll likely have your big battle of wits or heroics, or a violent battle.
B. The characters either fail or succeed.
C. Wrap up any loose ends, and end the story.
Review formatting rules. Formatting is only briefly explained below along with some writing tips. Below are some rules to know before you start your first draft.
The only transition you should ever concern yourself with is possibly "FADE IN." You do not have to use transitions between scenes. The editor and director ultimately decide which transitions to use. No use for Dissolves, Fades, Wipes or Cuts. Don't let transitions slow you down when writing. It's important to let your ideas flow onto the paper with little distraction.
Start with the tag-line. You first have to tell the setting and what time it is. If your scene takes place inside, the abbreviation is "INT" for interior. If your scene is outside, the abbreviation is "EXT" for exterior. Only use Day or Night. It doesn't matter what time, unless it needs to be specified. Your first your settings should look something like this: EXT. Forest-Night or INT. Basement-Day.
Set up the scene. Here you only really concern yourself with the action that is going on. If you are introducing a character for first time, you must capitalize his name. This is usually where you give a quick description of that character. It should look something like the following:
EXT. Forest-Night
The cover of the trees block the moonlight from any trail or way out. In the distance, we can hear twigs breaking where CHARLIE, mid-20s, short and dumpy, looks around with his flashlight. His eyes are opened wide as he searches for any familiarity. Suddenly, Charlie hears the whispers from the depth of the forest.
Write the dialog. Dialog is also a very important component of the screenplay. By using dialog, you can move the story forward, set a tone or mood. This is where the audience gets to know the characters. If the noise is off-screen, then specify that. All dialog is centered, including the character's name. So, now your formatted script will look like this:
EXT. Forest-Night
The cover of the trees block the moonlight from any trail or way out. In the distance, we can hear twigs breaking where CHARLIE, mid-20s, short and dumpy, looks around with his flashlight. His eyes are opened wide as he searches for any familiarity. Suddenly, Charlie hears the whispers from the depth of the forest.
Whispering (O.S)
We're here for you, Charlie. We want to kill you, Charlie.
Charlie
(frantically)
Who's there? What do you want?
String your scenes together. The above can be considered the first scene. Every scene should lead to the next somehow. If you were to start your scene right after the dialog, you would just put your tag-line right after. Again, do not worry about transitions, just continue your story.