Part three and the concluding parts four and five of The Night They Killed Kansas will debut next week as I take the weekend to craft it. In the meantime, please enjoy some writerly advice that I prepared for an interested party. I'm no expert and this is just my general opinion of getting started that I've developed up to now. My grasp of the situation will obviously change as time progresses, so indulge a young man's thoughts on how to develop into a screenwriter.
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The first thing I can say is that you have to want to write. It's not something anyone can just pick up and be good at. Whatever your writing, whether it be stories, scripts, plays, novels, poetry, nonfiction, etc. the only way to improve at the craft (and that's what it is, a craft) is to rewrite and rewrite. Great scripts can go through as many as 10-12 drafts by a single writer. And a draft isn't considered just fixing punctuation and deleting scenes. Drafting is about refining, honing, improving. Like any made-up recipe, it takes time, practice and a little bit of luck to perfect. And oftentimes, a writer never feels like she/has has "finished" what they wrote.
So, on the most basic level, just start writing. Keep a journal. Start a blog. Write more letters.
You don't even need a "GREAT, AWESOME, SUPER-AMAZING IDEA" to start. You could just write about your eyelashes, if you wanted. Exercise like you would a muscle, strengthen it.
Then, you can start to figure out how to express yourself through stories. If you're looking to write a screenplay or something, the key to one is not just about the idea/storyline/plot/high concept, but also how you express yourself.
Professional writers achieve success by having something to say and saying it in their own special way. If you sequestered three writers in a room and asked them each to write a two page scene involving two cats and a furball, you'd probably have roughly the same setup of the scene, but different ways of expressing the idea. In other words, your characters don't have to all sound exactly like you, but each of them, in their own way (as they are their own unique characters) will speak your mind to the audience.
Then there's always the other stuff about dramatic conflict, plot dynamics, escalation, etc. which are all structural notes that give shape to your Big Idea. Conflict is the essence of drama. Comedy, too.
You can pick up any number of books about structure.
Story by Robert McKee is a standard. But, if you are interested in writing, I say just start writing your thoughts down. Give yourself exercises and simple topics (like a teacher would assign an essay) and get to work on those. Really become confident in the notion that you could learn to be an even better writer before you tackle big time screenplays.
There are always classes and writers' groups and they're all helpful, particularly if your individual background lacks writing experience.
I'm sorry that your friend poo poos your ideas. It is easier to critique than to commend and easier to tear down ideas than encourage them, but, still, support will definitely make the process a lot easier.
If you have one or two movie ideas, I say develop your characters first and see how they fit in with your Big Idea. If you find that you want to have certain types of characters that don't jive with your Big Idea, then consider altering that Big Idea to fit your characters OR starting a new story altogether, one that will work better with the people you've created.