If you are going to write a screenplay, then you really should look into Celtx (http://www.celtx.com ).  Hands down, this is the best tool you can have to begin your creative process.  There are many ways you can write a screenplay, and many tools to help you do so.  But Celtx combines all the best features, and many that others don't have, into one product.  And one of the best things about it- it is Open Source!

It is based on Mozilla's FireFox web browser, and works on WIndows, Linux, and Mac OS X.