Fiction Basics: The Three-Act Structure
While it is true that many stories, television shows, and films don't always incorporate the classic three-act structure set forth by Aristotle in his Poetics , beginning writers would do good to learn about the basics of this classic story form. While successful stories can be told by altering or outright ignoring the classic structure, the three-act structure ensures a story has the basic "necessary" parts. For simplicity's sake, I will refer to the three acts as Beginning, Middle, and End.
Beginning: This is where the major exposition of the story occurs, where all (or most) main characters are introduced, setting is established, and characters start to leave their familiar world for an unfamiliar one. The beginning should comprise approximately 1/4 of the story.
Middle: Taking up more or less 1/2 of the story content, the middle is where the main action of the story takes place. Obstacles are presented and overcome, journeys are undertaken, friends and enemies are made, and (at the end of this act) the hero faces the biggest obstacle of all.
End: The resolution of the story, comprising about 1/4 of the story content. Characters return to their normal world (or, more rarely, resolve to not return at all), lessons have been learned, and whether everyone lives happily ever after or they don't, some final determination is made as to the fates of the main characters.
The classic three-act structure can be seen in the Monomyth, or the "Hero's Journey," a way of looking at all great stories and myths described by Joseph Campbell in "Hero With a Thousand Faces." While there are seventeen stages in the full Monomyth, they can easily be broken down into three sections (1. Departure, 2. Initiation, 3. Return) which closely correspond to Aristotle's three acts.