I wouldn't write it until you have possible publishers lined up. You can hit it a couple ways: Write a synopsis, and a "teaser" of the first few pages. That is all any publisher will read of an unsolicited script...if they open the envelope at all. Or write an outline of what you plan to write, and include any examples you have of a credited write (anything you've done that's been published in the past, even if only in the school paper). Send with a query letter to publishers that publish the type of books you want to write. Not all publishers work with every genre, so you need to find the ones that deal with your genre. It's a long slow road.
In the meantime, submit articles and short stories to every magazine you think you can write for. Again, start by sending a query letter. Learn to write a perfect query and it will go a long way to getting your foot in the door. Any publication you get will only help you.
at 4:58 PM on Jan. 31, 2009