When summarizing a story and picking out important events, what you are looking for are events that help move the plot forward, help portray themes, or help develop characters. If you can analyze a scene or chapter in terms of plot, theme, or character development, you'll then see if the scene/chapter is important, why it is important, and then be able to explain its importance. Since we are limited to space here, we won't be able to go over the whole book, but here are a few ideas to help you get started.
The first important event in the story is the moment we learn that Bingley has decided to rent Netherfield. This event is important because it serves to introduce the plot as well as the characters. We learn a great deal about both Mr. and Mrs. Bennet's characters in this first chapter. For example, we learn that Mr. Bennet is quite the sarcastic wit who really has no respect for his wife, as we see in his sarcastic responses, such as, "You and the girls may go, or you may send them by themselves," which of course would have been highly improper (Ch. 1) . We also learn the importance to the plot of Bingley moving to the neighborhood and are able to speculate that a romance, as well as a possible conflict will ensue.
The second important event is the ball that takes place at Meryton, as well as the conversation between Jane and Elizabeth afterwords. The ball is central because it is here that we meet Bingley as well as Darcy. It is also here that the conflict between Elizabeth and Darcy begins as Elizabeth observes him to be a conceited and prideful man. It is also here that Darcy delivers his famous snub, saying of Elizabeth, "She is tolerable; but not handsome enough to tempt me(it)" (Ch. 3). Also, in the conversation between Jane and Elizabeth after the ball, the love intrigue between Jane and Bingley begins to take place. It is in this scene that we learn just how much Jane admires Bingley.