There are two screenwriting computer programs that are used by 99% of the writers and filmmakers in the industry. Get one of them.
Don't try to save money on this item. It's much to hard to create a properly formatted screenplay without the assistance of a computer program.
If your screenplay isn't properly formatted it will scream "amateur" to the very people you want to impress--financiers, actors and knowledgeable colaborators. Bite the bullet and do it. It's half the cost of taking one screenwriting class at UCLA film school.
Final Draft and MovieMagic Screenwriter both claim to be the biggest seller and the most used in the industry. It looks to me like Final Draft is both a bit more modern and the more popular.
I use and recommend Final Draft but know someone who loves MovieMagic. Some highly successful screenplay writers actually use them both from time to time. They are both worthy tools and you won't go wrong either way. You can download demo versions of both of them to see which one suits your style.
You Can Help Keep This Site Going: Some of the companies whose products I recomment pay me a small commission if you buy them through my links. So, please buy through my links. I only recommend products I have personally reviewed and/or own and believe them to be worthy of your consideration.