What happens in the beginning of the book?
By "beginning of the book" I am going to assume that you would like events that happen during September. The book doesn't have a numbered chapter system. Instead, each chapter is the name of the month that the book is taking place during. The entire novel takes readers through Holling Hoodhood's seventh grade year.
One of the first things that readers learn about Holling is that he isn't Catholic or Jewish. He is Protestant. That isn't a big deal in and of itself, but Holling is the only Protestant in his entire school. This makes Wednesday afternoons a bit awkward for Holling, because all of his classmates either go to Hebrew School or Catholic Catechism. In order to put Holling somewhere, the school administration arranges for him to spend Wednesday afternoons with Mrs. Baker. Mrs. Baker is not warm and welcoming at this point in the book, so Holling assumes that Mrs. Baker hates his guts. Mrs. Baker even tries to have Holling retake the previous year's math class, so she doesn't have to deal with him. That fails, so she makes Holling do menial tasks like cleaning desks and chalkboards.
Chapter one also has a great recess event. Holling is out during recess trying not to be noticed by older kids like Doug Switeck's brother. It doesn't work, and Holling is pulled into a pick up game of soccer. Holling is tasked with guarding the older brute of a boy. Doug Switeck's brother tries to run down Holling in front of the goal. At the last second, Holling sidesteps out of the way and trips the bully. Doug Switeck's brother goes flying toward the goal post and hits his head super hard. Everybody is wonderfully impressed that Holling could "take out" the older boy.
Finally, chapter one introduces readers to Holling's home. The house may look nice on the outside and the inside, but readers learn that Holling's home life is not good. His mother is a weak willed secret smoker. His sister is antagonistic to just about everybody, and Holling's father only cares about himself and his business.