If you’re a writer, or a writer director or a writer-director-producer, or simply a producer working with a writer, sooner or later it would behoove you to purchase some professional screenwriting software. To the best of my knowledge, the industry standard hovers between Final Draft and Movie Magic Screenwriter. I personally prefer Final Draft. But another friend of mine prefers Screenwriter.
If you’re an indie producer on a budget, setting out to write your first feature, then you can probably find any number of free software programs on the internet. Celtix seems to be popular, but it does have some limitations. To find this and other free software programs, simply do a Google search.