What kind of video? If it's just something small, it doesn't really need to be proper unless you're wanting to learn to write scripts for bigger projects you might do. Otherwise, notes on a pad of paper might be all the script you need. If you decide you do need a real script and want free software, my vote is Trelby. If you have constant internet access where you write, Adobe Story is a great option, and you can upgrade to the full, offline version when you're ready. It has some great features for scheduling and such. Celtx has pros and cons. It's biggest pro is that it can diagram a set so you can plan lighting, camera moves and all that to you heart's content. The biggest disadvantage, is that when you want to print, you need to Typeset first, which basically means sending the script to their server to format into a PDF for you. I used Celtx for years, but personally, I wouldn't go back. It's way too bloated now.