What do you find is the most crucial in the plot in Chapter 1?

In chapter 1, Nick Carraway establishes himself as a reliable narrator. The novel starts with a piece of advice given by Nick’s father that basically states you should not judge people because you do not know what their life has been like. This introduction to the narrator’s life is also a piece of foreshadowing by the author Fitzgerald. By giving this advice to not only the narrator, but also the reader, we are unknowingly set up for the fantastical character, Gatsby, who is introduced later.
Yes, setting the scene here is also important. However, introducing the man that guides us through this classic tale set in the 1920s is the most important part of chapter 1.
In chapter 1 there is good solid background information that lets us, the reader, get to know the person telling the story. It establishes Nick as an outsider to the unique setting of not only New York, but also East and West Egg. Even though Nick gives a gentleman directions and feels like he is “an original settler,” he remains an outsider for the rest of the novel. Which is a good thing because the characters he interacts with are “careless people” sans Gatsby himself.

Arguably the most important element of the plot in Chapter 1 is Nick's introduction to the social world, in which he will move throughout the rest of the story. After arriving on the East coast, Nick quickly notices the difference between West Egg—inhabited by nouveau riche arrivistes such as Gatsby—and East Egg, where the old money elite live. This tension between old money and new is an important one that will develop throughout the story, most notably in the faltering relationship between Gatsby and Daisy Buchanan.
As we're introduced to an unfamiliar world of wealth and glamor, it's vital that we have someone who can make sense of it all, with whom we can feel some kind of connection. And that's the role of Nick Carraway. His Midwest background and common sense allow him to provide a commentary on the faintly ridiculous world in which he's now arrived. Like most readers, Nick is fascinated and repelled in equal measure by the East and West Eggers and the lives of opulent boredom they lead. In Chapter 1, he provides us with a way into their little world, but without going too far. We're being eased ever so gently into their natural environment, the better to prepare us for what will happen later.