Writing scripts is relatively difficult, unless you have a book you are writing the script for that has excellent dialogue. It is also imperative that you have screenwriting software, and although there are several free options, Final Draft is the industry standard (at least in the U. S.).
I have worked in the theatre - one of my degrees is in theatre - and I know that it is one of the most fickle industries there is (although I think writing the traditional way with agent and publisher is probably similar). In the theatre, it is about who you know. All the parties you read about that the celebrities attend are actually networking opportunities that no serious theatre person would miss (including me). It may be your next meal.
Because of the high pressure of the theatre, I normally would not have taken a screenwriting job. But one woman wanted me to write her story and invited me and only me to do it. Then she hired me and is paying me a decent amount of money to do it.
However, like most jobs, I was expecting a book that was well written to transfer into a screenplay. I was not so lucky. The story itself is interesting, but it is very difficult to follow. It is a memoir and that makes its marketability even more difficult. But this woman is paying me well, so I found a way to make it marketable. I only hope my writing skills will live up to the task.
In the process, I have learned more about screenplays than what I knew before. Oh, always knew that they had these persnickety rules (which Final Draft does for me), but I did not know why they have them - 1 page of screenplay = 1 minute of screen time. Really. I tested it.
If you are thinking about writing a screenplay for yourself or someone else, follow the rules. If you do not, when it arrives on the desk of an agent or producer, it will go in the trash. The other thing about screenplays is they are formatted to be read quickly and easily. If you don't follow the rules, the producers aren't going to waste their time.
One final note: if you write a screenplay, get it copyrighted and registered - immediately! This is one of the mot important things you can do. It is dog eat dog and you do not want to be the dog that gets eaten. This is one thing that does not cross over so much into the writing world. In writing, you generally do not need to copyright it unless you are planning on self-publishing.