Final Draft has always been my favorite scriptwriting application. It is quite easy to use. It also has some convenient tools like "screenplay compare" that highlights the difference between two drafts of a screenplay. However, Final Draft is a bit expensive. It costs about $200 or more.
Celtx is free and can be downloaded from their website: celtx.com
Long time ago, I remember writing a full feature screenplay in MS Word, which is a reminder that, with patience, you can do it by tweaking margins and spacing, but you're better off with Celtx (which didn't exist a few years back).