Create a catalyst for the story. Something needs to happen in the first 10 pages that sets everything off. For example, in "Independence Day," the catalyst happens when the aliens arrive.
Create a "Big Event" your character has to react to. This event usually closes the first act and should happen by page 20-30. The "Big Event" is usually a life changing event that forces the character to fully commit to a goal or need.
Create a "Pinch" and "Rising Conflict." The "Pinch" happens at the centerpiece of the story. It's a twist that forces the character to take more drastic actions, thus creating a "Rising" conflict for the final act. For example, in "Ghost" Patrick Swayze's character is murdered (catalyst), he meets the Whoopie Goldberg character who can hear him (big event), and he finds out that his best friend is the one who had him killed (pinch).
In the final act, the character must confront the crisis that serves as the bleakest point in the entire story.
Create a showdown or resolution. This is where the character (usually) finds a way to overcome the crisis.
Create your characters. Every well written character should have dominant traits, quirks and imperfections, but most importantly some kind of goal. He should also have a need that is blocked by a flaw, and a backstory behind that flaw.
Begin your screenplay with "FADE IN" and end it with "FADE OUT" or "CUT TO BLACK." There are exceptions to this rule where you can begin with something like "BLACK SCREEN" as in "The Butterfly Effect" which opens with a quote over a black screen.
Begin your scene headings by determining whether it's an interior or exterior, what the location is, and what time of day it is. Example: "EXT. HOUSE - DAY" and "INT. CAR - NIGHT."
Center your dialogue text beginning with the character's name with her words beneath it.
Keep your action within a scene direct and to the point. To show someone is sad, you would write: "John pauses for a moment, then slowly breaks down as he begins to cry."
Use the following format for flashbacks, dreams, phone conversations, and notes/messages: Begin with "JOHN'S FLASHBACK - INSERT LOCATION HERE" followed by the action beneath it. End the line with "BACK TO PRESENT" or "BACK TO SCENE."
Use a screenwriting software program like Final Cut or Movie Magic Screenwriter. You won't have to worry about margin rules, fonts, and so forth; all of that is an automatic default in the programs. It will also help you format the script.
Edit the script. Run a spell check and double check your grammar. Read it out loud by yourself or with a friend to see how everything sounds.
Create your title page. The screenwriting software will automatically mark where everything goes. All that should be included is the title in CAPS with who it's by below it, all in the very center of the page; your copyright year on the lower left followed by your name; and your name, address, phone number, and email all in the lower right.