If, like many people, you labor under the idea that for "real" writers, the plot comes effortlessly, dismiss that illusion now. While some writers were born with a sense of how to tell a story effectively, more of them do study the elements of plot and pay serious attention to how other writers successfully construct a narrative.
Getting Down to Writing Basics
Playwrights have this stuff drilled into them, but fiction writers often get away without basic instruction in what makes something dramatic. It's not magic. The elements of a good story can be studied and learned.
In fact, you've probably already studied them in your high school literature classes. It doesn't hurt to review them now, from the perspective of a writer and not a student. They may seem simple, but without them, your other skills as a writer—your ability to imagine believable characters, your talent with dialogue, your exquisite use of language—will come to naught.
Start, of course, with a protagonist, your main character. The protagonist must encounter a conflict—with another character, society, nature, himself, or some combination of these things—and undergo some kind of change as a result.
"Conflict" is also known as the "major dramatic question." Gotham Writers' Workshop puts it this way in their guide Writing Fiction: The major dramatic question "is generally a straightforward yes/no question, one that can be answered by the end of the story." What will happen to King Lear when he divides up his empire and estranges himself from his one faithful daughter? Will Elizabeth Bennet of Jane Austen's Pride and Prejudice get to marry for love, and will she or one of her sisters marry well enough to save the family from financial humiliation?
What sorts of changes do these conflicts bring about? Elizabeth Bennet learns the dangers of letting prejudice interfere with judgment. King Lear acquires humility and learns to recognize superficiality and sincerity. Both are wiser at the end of the story than they were at the beginning, even if this wisdom, in Lear's case, comes at a dear cost.
A story will hit various landmarks on its way from the story's beginning to the fulfillment of the dramatic question. The introduction presents the characters, the setting, and the central conflict. Involve your protagonist in that conflict as early as possible. Today's readers will generally not wade through pages of exposition to get to the point. Don't make them wonder why they're reading your story or novel. Hook them in the first page or pages.
From there, the character will face various impediments to the achievement of his or her goal. Known as rising action or development, this is part of the story's satisfaction. Readers like to see a struggle, like to feel as though the payoff at the end is deserved.
Again, Pride and Prejudice provide an excellent example. If Elizabeth Bennet and Darcy liked each other immediately, and their friends and family immediately approved, their marriage would be much less satisfying, and nothing much would have been learned along the way, except that it's great to fall in love.
Note how other writers build dramatic tension during this part of their narrative. How do they keep us interested in the outcome of the story? How many impediments are necessary to make the reader feel satisfied in the end? None of these decisions are necessarily easy. Part of your growth as a writer entails developing a feel for a successful story arc.
Rising Action to Tying up Loose Ends
The rising action leads to the climax, the turning point in the story, which in turn leads to the resolution. The central dramatic question is solved one way or another. Peter Selgin provides a good example in his book By Cunning & Craft:
Climax is the resolution of conflict, the point of no return beyond which the protagonist's fate -- good or bad -- is secured. Romeo's suicide is the climax...not because it's the most dramatic moment, but because it seals his fate and determines the resolution by preventing him and Juliet from ever living happily ever after.
In the denouement, the author ties up all the loose ends. Elizabeth and Jane Bennet get to live close to each other. Lydia stays far away in the North, where she can't bother them much, and Kitty's better qualities are drawn out by frequent visits to her sisters. Everyone, we like lives happily ever after, and in a matter-of-fact three pages or so, we get all the necessary details. Likewise, the denouement for Lear takes only part of one scene: all the players of the main plot die, but under Edgar, England is reunited.
First, much successful fiction does not follow these rules exactly. But even works like Virginia Woolf's Mrs. Dalloway, which seem focused more on language than action, introduce dramatic questions to keep us reading. (Will her party come off? What's up with her and Peter Walsh?) A lot of fiction that doesn't necessarily seem plot-driven turns out, on closer scrutiny, to depend on tried and true strategies we can trace back (in Western literature, at least) to Aristotle's Poetics.
Second, these basic elements may not occur in the order listed above. Try to identify them in your reading. Question why the writer decided to tell the story the way he or she did. Note the dramatic decisions. And, of course, think about all of this as you craft your own stories. At the end of the day, something has to happen. It seems elementary, but it can be quite complicated. By all means, experiment, but spend some time on the basics, too.