But, don't worry if you don't learn the skill quickly. I wrote three published mystery novels before I really understood that a plot is every important thing that happened.
I made the mistake, in my first three books, of including some things just because I knew them and others just because I liked them. Both are bad reasons.
What Works is the material that moves the story forward, or defines a character, or relays important information. If it doesn't do something, it probably doesn't belong in the book.
The next natural step after recognizing what doesn't work is not using it. However, you can save it.
However, the one big thing that stands in the way of your learning what works is the opinion of others who are not agents or editors.
Don't have other would-be writers or your family or friends critique your work.
We professional writers know that critiques by fellow beginners are not a good idea unless you are very insecure or plan to write only for fun.
With a critique group of fellow pre-published writers you are only getting the opinion of a bunch of other people who are ALSO unpublished; the visually-impaired leading the visually-impaired.
Critiques from your family and friends are even less helpful and more insidious because they all have an Extra Agenda. Your mom will love anything you write because YOU wrote it. That's what makes so many people think they can sing in karaoke clubs.
It does not mean these people do not love you, but they are totally unqualified to tell you if your writing is any good and they have a vested interest in not telling you the truth.
What's even worse is creating a dependency on critique groups or friends. It stunts your growth as a writer. It holds you back from learning the ability to recognize what works on your own, a skill best obtained by reading lots of books in your chosen category and by writing your own and asking lots of questions.
Recognizing what works in fiction is the most important things a writer can do. You can't write a book by committee. Every time you change something to please others, you water down your own unique voice and your style.
If you are getting critiques by professional, published authors, you might profit from their advice, as long as you do not rewrite to reflect each reader's opinion.
But, somewhere along the line you simply must gain the ability to tell bad writing from good writing without help from others.
Beginners getting critiques from other beginners is one of the reasons there are so many unpublished beginners.
Above from the book NOVEL SECRETS, available for Kindle for only $.99 cents: http://smarturl.it/lary