The three act structure is a common and effective method of plotting a story. This tutorial aims to serve as a straightforward and practical introduction to that structure, avoiding excessive detail and technicalities. For those interested in a more extensive overview, I recommend reading Alexandra Sokoloff's posts on the topic at The Dark Salon. (See the links in her sidebar.)
If you've heard a story described as a beginning, a middle and an end, you've already encountered the three act structure. The first act is the beginning, where characters and ideas are introduced. It's the first quarter of the story and ends with the first climax. The second act is the middle. It's all about conflict and opposition. It's also the longest act, at roughly half the story. As such, the second act contains two climaxes: one at it's midpoint and one at the end. The third act is the end of the story and leads up to the final climax. This act makes up just under a quarter of the story. By the end of it, we should see that the characters have achieved their goals or if you're of a more melancholy turn, that they cannot.
You might have noticed that each act culminates in a climax. (There is also a climax in the middle of the second act.) A climax is a point where the story's conflict comes into focus. In simplest form, this means that the hero must fight the villain. Difficult decisions and near-death moments are often involved, although conflict can take many different forms. The length of a climax will vary with the length of the story. While a paragraph-long climax can be more than enough in flash fiction, a novel might need one or more chapters to deal with a major climax. Each climax is more intense than the last, until the third act climax leaves the hero in a seemingly impossible position. It's not unusual for the last climax to make the hero give up, before he is spurred back into action. (Other climaxes can do this too, of course.)
With the three act structure, plotting becomes a case of creating four climaxes and connecting the dots. If you've ever struggled to fill up the middle of a story or felt that an ending was insufficiently exciting, the three act structure might help you. If your stories tend to meander a little, the three act structure can aid you in focusing them. If, like me, you would like people to stop talking about inspiration and imagination*, and to apply a little logic, the three act structure might be your new best friend.
At this point, you have everything you need to start plotting with the three act structure, but in the interests of clarity and education, here's a further breakdown of the different acts.
This is where we meet the characters: the hero, his best friend, the love interest and probably the villain. Some works may not have all of those characters and in a few genres (like whodunnits) the villain shouldn't be introduced until the end. Nonetheless, we should be aware that there is a villain!
By the end of Act One, the lead character should have committed to the main goal and the reader should know about it. The character should also have entered the story world. In fantasy, that might mean literally being transported to another world, but the change in world doesn't have to be literal. Your character might have finally started her new job, or just got divorced. The important thing is that there's been a change even if it's only psychological. These events often take place during the first climax. (At this point, the story is one quarter in.)
In the second act, things start to get serious. The character may have been playing around with a goal before, but now it's real. This is usually where planning, preparation, and training happens. It's also a good place to advance any subplots that you introduced early in the novel.
The midpoint climax usually takes all the planning and preparation of earlier in this act and throws it out the window. The villain might find a way to destroy a key component, or the main character might make a critical discovery. This is also the point of no return. The character was committed before, but there might have been a loophole or a back door. Now there's too much at stake to go back. The magic amulet that returns the protagonist home has been stolen; the hero's daughter is being held ransom; all the roads have been closed. (This happens halfway through the story, as the name implies.)
After the midpoint climax, there's a recovery period. The hero might wonder if it's really worth it, or if he has what it takes. However, the action should begin again soon. Plans need to be remade, and allies re-evaluated. The protagonist is usually thwarted at every turn, and urgency builds. Plot twists often play out in this section.
The tension peaks in the second act climax. Despite all the work that's gone before, or perhaps even because of it, conflict comes to a head once more. If there are any great revelations to make, this is often the place to make them. By the end of this climax, the outlook seems clear, and it doesn't look good. (At this point, the story is just over three quarters in.)
The character may need to recover from the last climax, but there isn't much time to spare. Everything in this act is directed toward the final climax it may even all be the final climax.
The final climax is the biggest of them all. The characters should have to face their worst nightmares, and change the way they look at things. There may be twists and turns. Things will look hopeless. The biggest events of the story take place.
Up until this point, every climax has been some kind of setback. This time you as the author have a choice. Comedy in the classical sense that is, a story with a happy ending requires that the hero achieves his goal and develops as a character. Of course, the whole story has been working towards that, but this is where it come to fruition. In tragicomedy, the character becomes a better person, but doesn't achieve her goal quite often, though, she may not even want it any more. In tragedy, the goal is not achieved and the character is devastated.
After the climax is a short section of resolution. They get married and live happily ever after; the king rules in peace now that the villain has been defeated; Daddy finally spends that quality time with his little girl. This is the point to tie up any loose ends before the story finishes.
That's it! You now have a map of the terrain, and four pins to mark the waypoints. All you need to do is mark the path but that's another tutorial.
*Inspiration and imagination are wonderful things, but neither of them has ever plotted a story for me.