there are Free Screen Writing Software available that you can use and try
out first before making your purchase.
There are also websites that
list screenwriting shareware and freeware. However, it looks like
with most of them, you may be on your own with these programs. And
you can get them at your own risk.
Reading how they work with
your word processing program makes it sound easy once you figure it all
out. Some are marked "not user-friendly." The
website also thanks the visitor for not emailing them with questions about
the software. If you don't want to spend the money with the costlier
software that come with all kinds of perks such as 24/7 technical support,
free gifts, and other free software, then these shareware and freeware may
be best for your initial purposes: http://scriptersworld.8k.com/swsoftw.htm
These are the more pricey
Final Draft: http://www.screenstyle.com/