How to write a film script

• The title of the film 
• Edit to start the scene (e.g. fade in)
• Scene number on the left hand side of the page
• INT. or EXT. depending on whether the location is interior or exterior.  Then describe where (e.g. EXT.  OVER MIAMI - DAY)
• Set the scene by describing the location and the action that is taking place.  You can also describe aspects of lighting and camerawork, but don't do this in great detail ( e.g. Stillness.  Long wispy clouds spider across a deep blue sky.  Sunlight glints expectantly off the mirrored high-rises downtown.)
• Character names should be in capitals.
• Dialogue should be placed in the MIDDLE of the page with the character's name (in capitals) on the line above, e.g.

                                                                   HEATHER BURTON

                                                   Ow, Todd!  You almost took my foot off!

• You can explain how a character says something by placing it in brackets before they speak:

                                                                   TODD BURTON

                                                  (under his breath)  Uch, your mother.  Pain in my ass.

• When the scene changes, state the transition on the right hand side of the page (e.g. cut to:)
• Remember to leave a line between each section of your script.  One page of a script is roughly equal to a minute of screen time.

NB  Use courier font for your script.

Here are some examples:

Here is some additional information about how to do it: http://filmschoolonline.com/sample_lessons/sample_lesson_format.htm