There are four things to consider when you start plotting your script: acts, scenes, pacing, and threads. Once I cover those, I’ll show you how to plot. Honest! Let’s deal with acts first.
An act is a section of the script that has the same basic direction, either up or down. Most plays and movies are based off three acts. A happy ending has a positive first act, a lot of stuff hits the fan in the second act, and the third act brings it all together. A tragedy reverses those directions, with an introduction to the loser’s life in the first act, a lot of cool, fun stuff happening in the second act, and the loser dying horribly in the last act.
It’s important to base everything off the basic three-act structure. Admittedly, a lot has to do with it providing a base that your readership will be comfortable with, and that you can far easier write from. At the same time, scripts with fewer acts don’t tend to work as well, and more acts just confuses people and are harder to write, as you need to look at the rising and falling actions more intensely.
[A rising action is an action with a positive connotation, and a falling action is one with a negative connotation. Big ones define acts, small ones define scenes.]
By breaking it down into acts, you are better able to organize your plot. However, the biggest problem you have is that you need to decide what the act breaks are going to be. These are scenes that are major changes in the flow, positive if the act is negative, and negative if the act has been positive.
Consider your standard action movie: The first act break is usually when the hero is having a great day, and is then forced into a defensive action by the villain. The second act break is when the hero takes the offensive, eventually winning the day. In a tragedy, the hero is losing the battle, and then gets the villain on the run (first act break), and then the villain starts winning (second act break).
In a romance movie, you have a couple just starting to get into each other, and then the first act break forces the romance apart. The second act break is usually when the lovers start coming back together. Unless it’s a tragic romance, in which case they are falling apart, something forces them together, rekindling the romance (first act break), and then the romance starts falling apart again (second act break).
Your first act should be the first fourth of the script, the second act should be the next half of the script, and the third act should be the final fourth. This makes a lot of sense, as the second act is the meat and potatoes of the script (making the first act the appetizer and the third act the dessert; these are apt analogies).
Your act breaks define the difference between acts. Your acts form the basic frame from which you will hang your plot, and need to be a major part of your plotting. But these are just beginning of plotting; scenes are the next consideration.