If, like many people, you labor under the idea that for "real" writers, 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.
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.
Elements of Plot
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.