Get a copy of professional screenwriting software such as Final Draft or Movie Magic and install it on your computer. Open a new document and choose "Screenplay" format. There are several selections such as television sitcom, play and teleplay.
Type a slugline. The slugline starts with "INT" or "EXT" which stand for "Interior" and "Exterior," respectively. Then continue on the same line with the location. Type a dash (-) followed by the time of day (Morning, Day, Night). The entire slugline should be in capital letters, for example:
EXT. SUBURBAN RANCH-STYLE HOUSE - NIGHT
Press "Enter" and skip down a line. Begin writing your action lines. The action lines describe the setting, introduce characters, describe the character's actions and what goes on in the background when it is important to the scene. The first time you introduce a character in a script, put his name in all-capital letters. Include important information about the character such as age, race and distinguishing characteristics.
Type the character's name in all-capital letters in the center of the page for dialogue. Then begin the dialogue on the following line. Indent the dialogue 1/3 of the page inward and only take up 1/3 of the page widthwise (so 1/3 of the page is blank on either side of the dialogue).
Alternate between action, dialogue and new scenes as you write the script. The screenwriting software should do all the proper spacing for you, and number the pages automatically.
Proofread and edit the script as needed, and save it.