A key tip about how to write a screenplay is to have something exciting happening every ten pages. This is one of simplest and most effective tips about writing a screenplay. Every ten pages, have something that shakes up the script a little bit happened. It does not mean that you have to start bumping main characters off every few pages (except you're making a horror movie, in which case, good luck and God bless), but it does mean that you have to keep the story moving forward.
Think of it this way - if you're describing a movie that you just watched to a friend who's never seen it, you do not focus on every little detail, right? You end up telling them the major plot points, the important stuff. My personal suggestion would be to try to write out your movie to yourself in a couple of paragraphs on one sheet - do not focus on the little stuff, the character minutiae and all the esoteric character jazz - just give yourself the plot, in its most bare bones form. Those events in the paragraphs that move the story forward? Those are the exciting things that you need to pace out through the script while you're learning how to write a screenplay.
I want to take a moment here to discuss structure in a broader sense, before we go any further - be very, very careful about how much of it you put into your script, especially when you're first learning how to write a screenplay . I would suggest thinking about it in musical terms - most people know that it takes a lot of skill to be a classical musician, but how many people regularly listen to classical music? Just because you can show off your technique or mastery of skill does not mean it will go noticed and be appreciated by an audience. If you need further proof of this, let me remind you that "The Big Bang Theory," the number one sitcom in the world is, from a structural standpoint, one of the worst technically written things on television right now yet it says much else about the overlooked aspects of how to write a screenplay.
You have to keep in mind that while you, and other writers and film buffs, may have an appreciation for your brilliant Act 2 setup or your excellent dénouement, the reality is that the vast majority of audiences have no clue what the hell those terms even meaning - they're in a theater to be entertained, and if you're too strict with your structure, they'll see everything you've got laid out for them from a mile away, and then nobody goes home happy.