Honestly, you can write your screenplay on anything from Microsoft word to Google Docs. But a properly formatted screenplay is designed in a way that one page equals one minute of screentime. this way you can get a much better idea of how long your movie will be. Some screenwriting software can be expensive, but there are plenty of free programs you can use:
Celtx is the software I originally started using. It’s super easy to use and a great one to start with for any new writer. The web version has a monthly fee, but you can still download the free version on to your computer.
Next up is Plotbot.com. This is a browser based program that I’ve been writing on a lot recently because I can access my screenplay from anywhere on any device (Specifically my Chromebook). Also within plotbot is a huge community that can give you feedback on your script.Here are a couple more that I have not tried but look promising: