The three-act structure is a method of storytelling with well-defined phases. There is the beginning, the middle and the end.
This is the most classic way to develop a story that will make sense in the minds of your readers.
The second act is usually much longer than the others, and can be treated as two sections divided by a climactic middle point. This template covers the basics, but feel free to add as much detail as you like.
Set the tone, the general atmosphere and introduce one or more characters.
Write one or more scenes that introduce the world in which the secondary characters are developing, and that hint at changes to come.
The unexpected moment that turns the character's world upside down and kicks off the storyline.
How do the characters react to the triggering incident? What choice does the protagonist have to make?
How does the main character decide to deal with the problem? What is at stake?
As the story progresses, troubles pile up and decisions get harder to make. Ask yourself what keeps the characters going in this vicious cycle and write it down. Your readers need to understand the turmoil in the protagonists' minds. At this point in the story, the main character usually still has his or her initial enthusiasm.
A surprise event that destroys the plan and raises the stakes, often after a temporary triumph.
After this turning point, we should feel that the problems are piling up, the main character can't handle it anymore, he has a knife to his throat. We feel the first signs of weakness.
It's the last straw. The protagonist(s) are trapped, facing uncertainty, a great moment of loneliness or an impossible decision. At this point, the readers think that the hero of the story will not make it.
A new hope is born. Usually thanks to an innovative idea from one of the characters. This glimmer of hope prepares for the final act and revives the story.
This is usually where it all falls apart. We feel hope being reborn and the plot unraveling. The pieces of the puzzle finally seem to fit together and the stakes reach new heights.
The protagonist faces the antagonist or the main obstacle and is victorious... or fails.
After this moment of great tension, the main character or the world in which he or she lives has changed. The page turns and you either decide to conclude with this ending or you use the final scene to set up the next part of your story.