The plot is one of the most important things to consider when you're writing your book. It is the driving force for all of your characters and gives the reader something to follow along with. For many authors, the plot is the first thing they come up with when they are considering a new story.
Step 1: The first idea
The first idea can be something super small. For some, the first idea can be that they want the story to be centered around a single character or multiple. The first idea can be as simple as "I want this person to save the world" or "I want character 1 to be against character 2 from the start and they are forced into a situation that makes them work together".
The first idea can also be a case where the author wants to write a book similar to one they love/loved as a child, but make it their own.
Step 2: Picking your genre/audience
This seems a little obvious, but the genre is very important. With genre, there are certain aspects of the plot that would work in one genre but would be completely out of place in another. Usually, advanced technology would not be in a High Fantasy book and elves and magic don't go in a Sci-Fi book. Of course, there isn't a rule against making your story different or even changing your ideas away from the norm, so don't let this discourage you!
As for the audience, you wouldn't be writing a book with adult characters with large amounts of cursing and sexual tones as a Young Adult novel and you wouldn't be writing a whimsical fairy tale for college students. Finding a balance is important!
Step 3: Solidifying your beginning, middle, and end
Making sure your characters have a point A to leave and a point B to get to is super important, but the middle is what makes or breaks a book. The hobbits didn't just leave the Shire and end up in Mordor! They had to go through Rivendell, Gondor, and endless forests and adventures in order to finish their quest.
Step 4: Side plots/quests
Meeting a new character, facing a monster, having a computer glitch, joining the rebellion are all great side plots for your characters. It's how they overcome obstacles and interact with others that really makes a great book. The main plot is what the story is about, but no plot is complete without some bumps in the road.
Don't be afraid to change your plot! In fact, it most likely will change. The plot you start with probably won't be the plot you end up with. As you get to know your world and your characters, things are going to evolve and change. The person you intended as the villain might not be the villain at the end!
Writing out, mapping, and all forms of diagrams could be EXTREMELY beneficial as you go! It keeps your ideas organized and everything running smoothly.
Disclaimer: I am not a professional, this is just based on my experiences.