Come up with a list of all your major characters. List each character's personality, goals, weaknesses, strengths and physical attributes. Make another sheet where you list how each character is related to the other, their history together or lack thereof.
Write out the "arc" of your play. Most plays and scripts are in three acts. The first act helps to introduce the characters, sets the tone and introduces the main conflict for the play. Write a one or two sentence description of this act. The second act twists the existing circumstances and heightens the tension. By the end of the second act relationships should be shaken or changed. Write a two to three sentence synopsis of the second act. The third act is the resolution of the conflict. Write a one sentence description of the third act.
Expand the descriptions of each act into a one page synopsis. After you have done this begin writing your script. If the script is for theater, place the character's names on the left-hand side of the page. Put the stage directions in italics. If this is for television or film, center the character's names. Center the dialogue under each character's name. Do not center the stage or action direction.
Put together a staged reading after you finish your script. This will give you an idea of how the script sounds and can help you identify any problems that you might not have noticed.