So you have your novel and have what you've always wanted, a publication contract. (Thanks Muse;) But there's still that part of you that wonders what it would look like on the big screen. Well I'm going to tell you from experience if you want to adapt your book to a screenplay be prepared for a lot of hard work. From the writing of it to the selling of it there are a a lot of avenues and pitfalls along the way and one should no screenwriting is just as hard as writing a novel and just as a legitimate artform.
The first thing you have to know is your market. Hollywood loves genre films especially horror because of the low overhead high return. Like it or not that's why there are so many of them. Hey I respect the genre, I couldn't write horror if I wanted to, I have no idea where to start, but to those with a horror story Hollywood and the indie world love you.
Thrillers/suspense/mystery Hollywood loves you too. Basically any genre script has a chance if written well. The hardest sell however is the romantic drama. But if that's what you write do your research and find agents and production companies who specialize in that sort of film. There's still no guarantee you'll sell your script or find representation but then there was no guarantee you'd find a publisher for your novel either but you wrote it. Just know, Hollywood makes the publishing industry look like a veritable walk through the park. Having worked in both industries I have been burned by both sides but it happens more in the film world.
This is going to sound simplistic but in practice it is, as you all know much harder: write a great story. Not an idea. There were only seven original ideas and Shakespeare stole them all ;). Register your work with the WGAw.
Screenwriting software is important in that it saves you a lot of headaches. Final Draft, MovieMaker are a few. I use a free one, Celtx. Here's a link http://www.celtx.com/ to it. It's free and simple to use. For a more in depth breakdown of screenwriting formating join me at my Chat at 9PM EST at http://themuseonlinewritersconference.com/joom/index.php?option=com_content&view=article&id=14&Itemid=13
As for elements of a great script everything must be SHOWN. No internal monologue. ACTION and DIALOGUE are your only tools. Also, you have 90-120 pages to tell your story. If you're convinced you script should be 180 pages remember 1 page=1 minute of screen time as a rule and 180 pages=3 hours of butt time in a theater. If you're new at writing scripts find yourself someone who has had their material produced.
How do you do that? Search online. Check out your local indie film scene. Ask questions privately. But remember the film world is cutthroat and you might not get a friendly response. However you'll never know unless you ask.