Knowing how to write a movie script is not something that you just wake up and are magically good at - it's a skill (although foolish people will tell you otherwise), and in order to get better at it, just like any other kind of skill, you have to practice, practice, practice, and that doesn't mean doing it for a week and then saying "I'm done" - you have to treat it like you are learning a musical instrument - the more you practice, the more "in tune" you will be with yourself and your writing, and the better your scripts will be.
However, there is an essential, and often overlooked part of learning how to write a movie script, and that is actually reading a great screenplay. It is amazing to me how often people who want to be screenwriters will openly confess that they have never read a single screenplay, and they are only doing themselves a disservice by continuing to not read them - in order to know how to write a movie script, shouldn't you use a great piece of work as a template? This is an easy, easy thing to fix, and we'll talk about some ways below on how to do that.
The first, and most important thing here if you want to learn how to write a movie script, is that you have to pencil in the time to read them. It doesn't matter when you read, it doesn't matter how you read, it just matters that you read an agreed upon amount each day from a screenplay, preferably one of a movie that you really enjoy - figure out what, from a screenplay perspective, makes you so enthralled with that project. You may end up figuring out that you're really attracted to dialogue heavy scripts (this is especially common for comedy writers), or you may find out that horror movies are where you feel most comfortable - either way, you'll take away something from the experience about yourself and your personal tastes.
Proper planning and structure is also key to your success. If you're writing a movie script, you absolutely must understand three-act structure. If your script lacks proper structure, it will just be a series of events that do not move the plot forward. Receiving feedback from other screenwriters will help, but don't wait until you have written the first draft. Get feedback during the planning phase -- treatment writing.