There are a lot of different screenwriting programs out there. And for most of your screenwriting purposes, it's fine to use whichever you feel the most comfortable with.
However, if you're looking to break into TV writing, there is one screenwriting software that you MUST know because it's the only one used in any TV writers' room.
TV Writers All Use Final Draft
Whether you love the program or hate it, facts are facts. I've worked in 14 different writers' rooms with all different showrunners and writers.
Every single one of those writers' rooms used Final Draft. I only heard stories of TV shows using Movie Magic, but that small contingent had phased out years before.
Can't I just write with another program even if everybody else is using Final Draft?
No. When you write for a TV show, your script is not your script. That script will be passed around to the showrunner, maybe another writer to write a scene, and it will be noodled with incessantly.
And because that's happening, everybody needs to be able to use the same file with the ability to send it around as an attachment to anybody who needs it.
I'm Not Writing for TV Yet. Do I Still Need to Use Final Draft?
No, but...
I would strongly recommend using it to get the hang of it, or at the very least, finding a program that can convert files into FDX, which is the Final Draft file format.
WriterDuet is one such program that can do that, and it has a free version of their software.
But there are some other reasons I feel some of the other popular screenwriting software should be avoided...
Let's take CeltX for example. It used to be great... when it was free... and when you could work on a screenplay that you owned as a file on your computer.
Now, you have to log onto your CeltX account in order to work on a screenplay in your browser. This is a very scary reality if you're going in and out of internet range, like on a flight. Will this software save your progress? Sometimes.
Also, you can't access or send your file to just anyone. You have to log on and "collaborate" with someone who also needs to have CeltX. It's a huge pain in the ass.
Not to mention, the formatting is just a little bit off.
It's these little things that I've found consistently proving that Final Draft is the only screenwriting program you should use.