Interview as many people as you can who lived the story. Try to get different perspectives on the story. Record the interviews so you can refer to them later on as you develop your story. You can also get information from newspaper accounts or books on the subject if they exist.
Outline your story. True life doesn't mean your script should include every detail of someone's life or the events you're writing about. Most true-life stories are based on true-life events. You have to decide the focus of your story and what to leave in and what to leave out. The everyday events in someone's life should be disregarded unless they have a direct influence on the story. You'll have about 2 hours to tell the core story, so there isn't room for superfluous information.
Take advantage of creative license. The core story is obviously good enough to base a screenplay on, but that doesn't mean everything needs to be written exactly as it happened. You might change the timeline of events to fit them into your story smoothly. You may combine events or even characters to include them in your story without losing the forward drive of the script.
Format your true-life script as you do any screenplay. This means to format action in sentence case with paragraphs separated by a blank line. Write 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. CLUB - NIGHT. Page margins should be 1.5 inches on the left for binding and 1 inch everywhere else. Character dialogue headings should be written in all capitals and set with a left margin of 4 inches and a right margin of 7.8 inches. Write dialogue in sentence case with a left margin set of 2.8 inches and a right margin of 6.1 inches. Include in your script only what the audience can see and hear on the screen.
Use script formatting software such as Final Draft or Movie Magic Screenwriter. These programs handle the tedious formatting chores of the screenplay for you, which allows you to focus on writing the story. You can also use free scriptwriting software like Celtx (see Resources). Another option is to use online scriptwriting software. Scripped is free and works as well as stand-alone software (see Resources).