There are hundreds of different sites that have hundreds of different "how to" tutorials that will show you hundreds of different ways to write a screenplay correctly.
For the most part, they're all wrong.
If you want to be a screenwriter, you've got to know how to write a screenplay - formatting and structure
Screenplays are written in a very specific format. The entire document is divided into scenes, and the scenes are structured to be a slug line (the heading of the scene telling you whether the scene is inside or outside, a location identifier, and what time of day it is that the scene is taking place), description (or action) and dialogue.
My first recommendation is to read screenplays. You can do this here:
www.script-o-rama.com
www.simplyscripts.com
www.imsdb.com
You'll also need a good book on formatting. For that, I recommend the Screenwriters Bible by David Trottier. He explains all the terms and gives examples. It's a great book. You can pick it up cheapest on Amazon.com
You'll also need formatting software. There's the professional software packages: Movie Magic Screenwriter and Final Draft. MMS is the industry standard, and - of the two - I find that it's the easiest to use. There's also the free software. It's not a standard, but it's certainly the cheapest way to get going. The software package Celtx is probably the best out there. You can get it here:
www.celtx.com
The next thing you'll need to know is how to structure a screenplay. While the story is always up to the writer, it's important to know that virtually every story is the Monomyth. If you include the plot points outlined in Hero With A thousand Faces by Joseph Campbell, you'll be well on your way towards having a marketable screenplay. Joseph Campbell's book is a great reference, but really dry reading. For now, read (and understand) Vogler's breakdown, which was a memo written awhile ago. It's very concise and hits the high points you need to know to create a good story. You can read it here:
www.skepticfiles.org/atheist2/hero.htm
A screenplay is between 90 and 120 pages for a movie.