How to start writing a screenplay? And above all, how to write a good script?Watch
From a technical point of view, you can be very basic and use Word. Although, it's much easier to download software created for screenplay writing. Celtx, Trelby, and Writer Duet all have free versions iirc. This will ensure you can structure your screenplay a lot easier.
For actually writing - having a core idea or concept can really let your mind run wild and just invent as you go along. Don't worry about how bad it is. People give up too fast because they want it to be perfect. It will never be perfect. Maybe after the 10th time you revise it you may be happy.
Your initial idea could be 'bank robbery gone wrong' or 'woman rises in the fashion world' or you can take inspiration from your own life/interests. Understand what drives your story. Look into popular story types, like rags to riches, heroes journey, overcoming monsters. Do research, don't start a story in Egypt if you don't know anything about Egypt! Detach yourself from the story. Each character has their own past, present and future. You may absolutely believe in God. Learn how to write characters who absolutely despise the idea of God. Don't stay in your comfort zone and think bigger than writing about your rotten mates at University.
If there was a winning forumla to writing a good script then the industry wouldn't be so rich. For everyone one person that actually gets their script produced with a sizeable budget, there's 10,000 who didn't get ****. I would advise your first script to be something you could potentially produce yourself with a few mates. A short film. Oh, just so you know, each page in a screenplay represents roughly one minute of screentime when produced. You write a screenplay for a short film. 3 pages would be around 3 minutes long. Sounds easy? Try making those 3 minutes engaging. It's actually hard. My first short film I took inspiration from the opening getaway scene from Drive. I basically tried to recreate it with a couple of friends to see what was possible in a car as I was curious how we'd do it without all the fancy rigs. It was terrible, but fun.