Write your spec script with the story at the forefront. Don't write camera angles and try to direct the movie. Tell your story with words and leave the directing to the director. Only write a camera angle if it is essential to telling the story.
Stay away from scene numbering. Scene numbers are only used for shooting scripts and are distracting. A spec script never uses them.
Write only what you can see and hear on the screen. Nothing else exists. Write everything in present tense. Page margins should be 1.5 inches in the left for binding and 1 inch everywhere else. Scene headings are written in all capitals and should tell readers whether the scene is inside or outside (INT. or EXT.), where the scene takes place, and whether it's day or night. Here's an example:
EXT. BASEBALL FIELD - NIGHT
Precede all dialogue with a character heading. This indicates the name of the character speaking. Write the character heading in all capitals with a left margin set at 4 inches and the right margin at 7.8 inches. Write dialogue in sentence case with a left margin set at 2.8 inches and the right margin at 6.1 inches.
Avoid using parentheticals in dialogue. A parenthetical is a bit of information in parenthesis to indicate a small action or a character's expressions. You can use them sparingly, but try to make the dialogue speak for itself. Actors like to deliver the lines without being told how to do it.
The title page of your script should include nothing except the name of your script, centered, with a "by" line and your name below it. Include your name, address, phone number and e-mail on the bottom right hand side of your title page. Bind the script between blue cover stock with nothing on it. Don't use fancy fonts or graphics. Punch three holes in it and bind it with 1/4-inch brass brads. Don't do anything else to it. Fancy covers don't sell a spec script. Your writing should speak for itself.
Use Movie Magic Screenwriter, Final Draft, Movie Outline, or Celtx to write your script. These programs are dedicated screenwriting programs that will handle the formatting chores for you automatically, leaving you free to concentrate on telling your story. Celtx is free (see resources).