The problem many writers have when they try to write a screenplay is that they only know what a screenplay is from seeing a movie. But the movie isn't the script. In fact, it's quite difficult to "see" the script when you see the film. It's like looking at a building and seeing the blueprints. I suppose it can be done, but I think it's easier to learn how to make something by studying the thing you're trying to make. Hopefully, you're trying to make a screenplay. Once that's done, then you can think about whether you want to make a movie from that screenplay.
I began to teach myself how to write screenplays before I had the chance to go to film school or to learn on the job. How did I do it? (And the job of learning how to write screenplays is far from finished for me! I suspect it's a career-long task.)
I did it by reading the first two screenplays I found. This was the late seventies and published screenplays were NOT the norm in bookstores. But I found two: "Butch Cassidy and the Sundance Kid" in paperback and "Citizen Kane" which was included in "The Citizen Kane Book", also in paperback. In retrospect, I'm so grateful to the whoever guides us through life that I was given those two screenplays to begin my autodidactic lessons. I couldn't have asked for two better examples because they're both classics and they're both not "written" correctly! In fact, the Butch screenplay is totally in the wrong format! (Doesn't stop it from being brilliant.)
What both those screenplays DID teach me was how vital it was for the story to be told in pictures. Yes, there was dialogue but fundamentally, those screenplays are Moving Pictures.
The third book I read is also a classic: Eugene Vale's "Technique of Screenplay Writing". Vale's book was one of the first "how to's"--published long before the likes of Syd Field et al came up with their formulas.
I hate formula. I love structure. Vale understands the difference and I'm grateful that I read that book before someone ever shoved Syd Field in my face--which in my view is more formula than structure--especially for someone new to screenwriting. Screenplays are not fill-in-the-blank documents, though you could easily believe that from reading Syd Field. Field's books like "Screenplay" aren't wrong. But his approach schematizes screenwriting.
I suggest starting with screenplays first. Lots of screenplays. After you've read at least ten, then, maybe, find a screenwriting handbook to read with a grain of salt. It's important that you learn from the master writers, rather than shoehorn your nascent story into a formula that you may not understand fully.
If you really want to understand structure, go to the master of them all, Aristotle and read his Poetics. It's actually all there. But a tough slog if you don't know your way around ancient Greek theater.
Read screenplays to learn to write screenplays. It's kind of obvious, but the knowledge you gain from this method is knowledge you will own.