Start with a good premise and find the humor in it. Think universal. Find the humor in everyday events that anyone in the audience will be able to relate to. Write an outline for the story. It doesn't need to be a detailed outline. A simple step outline showing the major plot points in the story will do.
Develop characters that viewers can relate to. Think of some of your favorite characters and the types of roles they've played in the past. Adam Sandler is a master at playing underdogs who come out on top. Writing with specific comedic actors in mind will help you define the characters in your comedy script. Studios and the comedians are always on the lookout for good material. Focusing your writing efforts on specific comedic actors and actresses can give your script a foot in the door.
Leave room for comedians to do their job. Much of the humor in comedy screenplays comes from the way the comics play the roles. Facial expressions and ad lib dialogue are a big part of what a comedian brings to the script. Don't tell the actors what faces to make in a script, and leave room in the dialogue for them to add their own personalities.
Don't write camera angles. This is a spec script, not a shooting script. The director will determine how to shoot the film. A spec script should include action and dialogue. Nothing more. Write in standard script format. Write in present tense and only what you can see and hear on the screen.
Format action in sentence case with paragraphs separated by a blank line. Format scene headings in all capitals. A scene heading should include whether the scene is interior or exterior, where the scene takes place and whether it's day or night. A scene heading is written like this: INT. COMEDY CLUB - NIGHT. Page margins should be 1.5 inches on the left for binding and 1 inch everywhere else. Character dialogue headings should be set with a left margin of 4 inches and written in all capitals. Write dialogue in sentence case with a left margin of 2.8 inches and a right margin set at 6.1 inches.
Use Final Draft, Movie Magic Screenwriter, Celtx or similar dedicated screenwriting software to write your script. These programs handle the repetitive formatting chores for you, allowing you to concentrate on writing the story. You can download and use Celtx free of charge (see Resources).