The 3 act structure is one of the best tools to write a novel— even though it actually has 4 acts. I’ll get to that in second, but first, here’s a breakdown of the 3 acts.
Act 1: The Setting
The first quarter of a story is Act 1 and it sets up the world. Kick it off with an interesting first image or scene which sets up the story’s mood, texture, time, and place. The main characters, their situation and back-story are typically introduced in this act. Everything builds to a twist or turn in the story which propels our hero into a new direction, called a turning point, which kicks off Act 2.
Act 2: The Complication
Otherwise known as the middle half. Notice I said “half” and not “third” since I like to think of Act 2 as 2 separate acts. Obstacles are thrown at our hero until the halfway mark of the story where we hit another turning point that flips the whole story on it’s head. This leads into Act 2.5 where even more obstacles are thrown at our hero until it looks like he or she has been defeated. This is where he loses the girl or his best friend is killed or he loses the will to keep moving forward (or all of the above with some salt poured into the wound for good measure). That is, until he has an epiphany or a final piece of information is given to him. This is the final turning point that signals the beginning of Act 3.
Act 3: The Resolution
The final quarter of the story kicks into high gear, usually with some sort of countdown. A time bomb is the most obvious example or the hero has to go crash a wedding before his true love says “I do.” This leads to the big finish— and it should be a big finish. “Lets blow this thing and go home” take-out-the-Death-Star kind of finish followed by a short resolution that ties up any unresolved story threads.
There you have it in a nutshell, the 3 act structure in 4 acts. There’s quite a lot more to crafting the structure to your novel, but this is a good start to crafting your story.