What is the definition of a plot? Put simply it is a plan of the sequence of events your novel is going to take, which will form the basis of your storyline. 

This is an example of a sequence of events.

Helen woke from a deep sleep in the morning
She showered and brushed her teeth
She then drank a cup of coffee made by her husband
Once dressed, Helen drove to work 
She checked her emails and attended a meeting
At lunch, she ate a sandwich
In the afternoon she attended two further meetings
Helen drove home after work where she prepared dinner
She played with her children before putting them to bed
Helen watched some television and went to bed.

Intriguing? Interesting? I imagine that you would find a novel like that quite boring. That is because there is no plot. Nothing exciting or out of the ordinary happens. So, the first thing you must do when creating a plot is to have something happen that would draw the reader's attention.

Let's try the example of Helen again - 

Helen woke from a deep sleep in the morning
She showered and brushed her teeth
She then drank a cup of coffee made by her husband
Once dressed Helen drove to work 
Helen noticed that a car was following her 
Helen pulled into a parking space and noticed the car pulled in behind her
Helen entered her office
She checked her emails and attended a meeting
At lunch, she ate a sandwich
In the afternoon she attended two further meetings
Helen left work and as she arrived at her car, there was a note on her car windscreen
The note warned her that her life was in danger
Helen drove home after work where she prepared dinner
She played with her children before putting them to bed
Helen watched some television and went to bed.

Do you notice the difference? Now you have an engaging incident. This incident will form the basis of your plot. This incident will give you options as to where you take your story. Who was driving the car following Helen and why is her life in danger? The possibilities are endless, and the choice as to where you take the story is down to you, the writer. I would like to emphasize, though, that a plot doesn't necessarily have to include any action. The incident could have happened inside Helen's mind. Her fears, thoughts, beliefs, and motivations in a logical sequence would make for an equally engaging plot. 

Storytelling requires a protagonist and an antagonist. In the first act, you create a problem/conflict. In the second act, you resolve that problem and in the third act, the protagonist discovers his actions did not fully resolve the problem, and further action is needed. That is the most simplistic formula. You can, of course, add more conflict and the majority of stories have multiple sources of conflict. There can be more than one antagonist or protagonist if you give them henchmen and a sidekick. There are no hard and fast rules but there is a template to which most plots adhere. 

The decision that a writer needs to make is whether to plan out each detail before writing, or just begin to write and see where the story takes you. The choice you make is down to your personal preference, but you need to remember that every action a character takes will take your story in one direction or another. So if your plot isn't planned a little you could find yourself down a blind alley with no means of escape. 

So what is the answer?

For me, I first write a rough outline of the idea. Then I write a brief description of the characters, their physical description, their work, their lifestyle and any behavioral characteristics. I then give some thought about how the characters would react to certain situations. Next, write a brief synopsis and decide how these characters could interact with each other. What problems could arise between them? What complications or obstacles could you add to flesh your plot out? You will find that the characters will sometimes take over and have a life of their own. But remember, that every action they take must be believable to the type of personality they have. Are all the loose ends concluded? If not, go back to the point in the story where that thread of plot began and see if it needs to be changed. 

Finally, if you get stuck in a certain part of your plot, then don't get disheartened and do not try to fight it. Just leave the story for a few days and return to it; you may find new inspiration and ideas come flooding to you. I have found this happens many times. Above all, keep writing and allow your imagination to take over.