free script writing software

Free script writing software is definitely available.  Some are freeware and shareware and in most cases, there is no technical help with these software.  You must figure out how to use them on your own:  http://scriptersworld.8k.com/swsoftw.htm

However, you can use these more expensive ones for free--the demo versions only--if not to buy, to at least look at actual screenwriting format and apply what you learn here to your free software.  These offer 24/7 technical help:

Movie Magic Screenwriter

Final Draft

Sophocles

Screenwriting Index

Amazon