You may have heard that a novel is not subject to the guidelines of the three act structure because “it isn't a play”. I could go into a long explanation about the development of three act structure prior to the printing press and if books had been in mass production at the time it might be called something different. But I think it's best to keep it simple.
The foundation of three act structure is this: every story has a beginning, a middle and an end. In fact, if any story that doesn't have a beginning, middle and end is probably not a story at all but a different type of prose I like to call “unfinished”. That, I think, should be an argument for another day.
Much of the confusion surrounding three act structure comes from a lack of understanding about what exactly a “beginning”, “middle” or “end” looks like. So, lets break it down, starting with: The Beginning.
It is easy to assume that the beginning of a story is also the beginning of the main character's life. Or, at the very least, the moment they wake up on the day you decide to start chronicling his/her life. In a few cases this might be true.
More accurately, the beginning of a story is the point at which (or immediately before) the main character encounters a challenge that cannot be ignored but must be resolved – either through overcoming the challenge or succumbing to it. This is commonly termed the “inciting incident”. Some stories require you to provide context for the inciting incident, but in general you want to start as close to the moment the MC's life changes forever as you possibly can.
In some cases the “challenge” may be presented as a goal which the MC must achieve by overcoming obstacles. It must still be something that cannot be ignored. In other words, the challenge or goal must be something that has a direct impact on the MC's life.
A few common inciting incidents:
Boy meets girl/girl meets boy and falls in love but the girl/boy is out of his/her league or engaged to someone else or going out of the country the following day.
Something goes terribly wrong – this covers just about any story about alien invasions, horror, mystery, natural disaster.
An average Joe stumbles onto a conspiracy/learns that magic is real/learns the world isn't real/etc.
Someone is killed as payback and the Main Character swears revenge.
Notice how most inciting incidents also give us an idea of where the plot leads? Remember, a good inciting incident is a challenge that cannot be ignored but must be resolved. It is a goal that drives the story. (And yes, this is still true even for character driven stories.)
In general, the beginning of the story should present the reader with your Main Character. It should give us some insight into his/her current circumstance. It should introduce a challenge or goal for him/her to overcome/achieve.
If your beginning includes those basic elements, you are well on your way to finishing a good story.
So, what have you written today?