Writing a screenplay entails following a fairly complicated set of rules about tabs, indents and spacing, so a word processor that can handle all of that for you is a big help. The industry standards are Final Draft, and Movie Magic Screenwriter, but they're expensive if you're just starting out. Celtx is an open-source media preproduction environment that isn't quite up to the level of Final Draft and Movie Magic, but it's good and getting better all the time. And it's free.
Download and install the application from celtx.com. Celtx is a free program with versions for Windows, OS X and Linux. Make sure you're downloading the latest version for your operating system and in your language. (There are versions localised into several global languages.) Downloading and installing is a fairly straightforward process of following the instructions and clicking on buttons.
Once you launch Celtx, you'll see the welcome screen and a list of project types. In addition to a film template, Celtx offers templates for stage plays, audio plays, comic books and more. The trick to using Celtx properly is to ignore all of that razzle-dazzle and focus on what you want to do right now, which is write a screenplay. You can always come back and explore the other stuff later.
Celtx does a very good job of figuring out what kind of element you want to use next. But it's not perfect. It will start you with a scene heading; you can tell because there's a grey line across the screen. Also, look for a little text box among the icons atop the screenplay window; it should read "Scene Heading."
"This is all well and good, but the first thing you want isn't actually a scene heading. According to Hollywood tradition, you always begin a new screenplay with FADE IN:. And that's a transition. So pull down the menu that says "Scene Heading" and select "Transition." Note that you're now flush right. Type the magic words "FADE IN:." You'll also note that you don't need to hit Shift or Caps Lock here. As elsewhere, Celtx will automatically capitalise elements that should be in all caps according to screenplay format.
When you hit return after FADE IN:, Celtx will choose the next element type for you. What should come after a transition is, again, a scene heading, so the grey bar should reappear. Type a slug line (something like, EXT. MEADOW - DAY). Again, Celtx knows this should be capped and takes care of that for you. When you hit return this time, instead of creating another scene heading, Celtx knows it should be followed by an action element, and creates one of those. You're now into your first scene. Celtx is very good at figuring out what should come next and taking care of the formatting. All you have to do is create.
So far, we've seen situations where it's obvious what should happen once you hit return. But at other times, you'll have a choice. Once you've finished a paragraph of action, for example, you could add more action. Or you could have a character speak. Or you could begin a new scene. Celtx assumes you want another action element. If you want to start a new scene, just hit Return again, and you're back in Scene Heading. If you want someone to speak, hit Tab, and Celtx will automatically set up a Character Name for you.
After the Character Name, you have two choices. The most common is dialogue, so that's what Celtx will automatically give you. You could, however, want a parenthetical there. (This is an aside to an actor him how to deliver the line.) If you want one of these, just type the opening parenthesis, and Celtx will instantly realise what you're doing. When you hit Return after the line of dialogue, Celtx sets you up for another Character Name. If you want to go back to Action instead, hit return. You can almost always get what you want by hitting return or tab. If you get confused, look at the pull-down menu atop the screenplay window. It always tells you which type of element you're in at the moment, and you can always manually choose what you want from there.
In addition to just formatting your script, Celtx contains a wealth of additional features. Again, these will quickly overwhelm you if you just dive in. But Celtx will handle some of them for you automatically. Every time you start a new scene, the slug line for that scene appears, numbered, in a box in the lower left. As your scenes start to pile up, you can use that box to navigate through your script.
Another example is the Master Catalog of characters. Above the "Screenplay" item in the Project Library box (in the upper left) is something called Master Catalog. Every time you have a character speak for the first time, that character gets added to the Master Catalog. If you open the Master Catalog you'll see an entry for each character, where you can take notes, clip in pictures of actors you imagine playing the characters ... and generally waste a lot of time that could be spent writing. You may find these helpful, or they may just get in your way. Once you're familiar with the main writing interface, there are all kinds of extra features to explore if you want to.
You can save your file as a Celtx project, but apart from keeping it on your hard drive, there's not much you can do with a Celtx project file. If you want to send someone your work, you're probably going to want to export it to a .pdf. PDF is sort of the new lingua franca of Hollywood, and anybody who wants to see your script will probably want it in that format.
One of the tabs along the bottom of the script window reads TypeSet. It's essentially a print-preview window that shows you what your script will actually look like on paper, complete with page breaks. It's not editable, but you can save your script as a .pdf from here.
As noted, there are a ton of features to explore in Celtx. But you can produce a complete script using just these core features.