What happened in the beginning of the book?
This depends a great deal on what you mean by the beginning of the book. I'll say a little about what happens in Chapter 1.
In this chapter, we see only two people -- George and Lennie. They have just made their way to a pretty spot near the Salinas River. They are going to camp there for the night on their way to a ranch where they will work.
We find out that Lennie is carrying a mouse that he has accidentally killed while trying to pat it. George complains about how stupid Lennie is for a while. But then he stops being angry and tells Lennie the story about the dream the two men have.
In the beginning of the book George and Lenny are walking down a path that has been beaten down by tramps traveling through it. It is a hot day on the bus had dropped them off miles ahead of the ranch where they are hoping to find work. They come to a green pool of water and Lenny drops down and drinks. George scolds him not to drink so much. Then he tests the water and gets his own drink.
As the previous narrator stated this is the opening scene in which Lenny has a mouse hidden in his pocket. It is the foreshadowing of things to come. The scene sets the place that Lenny and George chose in case anything bad should happen during their stay at the ranch.