Act 1 in a trilogy or series is book 1. The author establishes the new world the protagonist will discover and what this world has to offer.
Something about the protagonist’s life makes him unhappy or feeling out of place, but he’s not yet ready to do anything about it. He usually has an open mind but sometimes he is also thrown into this new world against his will. (Think The Maze Runner)
The hero is presented with an opportunity to change his life (inciting incident), so he investigates it, but ends up being dragged into it. The world in which he steps in is made clear to the reader.
The protagonist spends most of the book discovering his inner strength and fighting his weaknesses.
EXAMPLE: In Twilight, we know that Bella is unhappy because she feels like she does not belong anywhere, she’s awkward and wants something more out of life. She is introduced to the vampire world, and invited to change her life. She embraces the new world wholeheartedly and convinces Edward to let her try it. She also learns that she is in deadly danger because she is human.
EXAMPLE: In Alyson Noel’s The Immortals, Ever is unhappy because not only her entire family died in a car crash, but also she has acquired strange powers. She tries to keep apart from people so that she will not read their thoughts and feelings. She is introduced to the world of the Immortals and must decide if she wants to be part of Damen’s world after an Immortal tries to kill her.
At the end of book1, we have the feeling that this is just the tip of the iceberg and there is so much still to discover. The heroes realize how much farther they still have to go before they can live in peace.
EXAMPLE: Edward realizes Bella will never be safe when he is around, and Bella realizes that if she stays with Edward, she will put her family and the persons she loves in mortal danger. She's ready to take the risk; he is not.
At the end of book 1, a key question is asked that will be answered in book 3. This key question is raised by the inciting incident.
EXAMPLE: In Twilight, the key question is will Bella survive? The question is raised because a vampire has just tried to kill her and she knows more will come and try to kill her again.
Johnson, Elana. "Writing a Trilogy, Part One: Book One."