When you are writing an essay there are several different types including, research, reflective, and informative essays. As you state you are writing about a book by Judy Blume you will most likely want to write a reflective or informative essay.
A reflective essay is an essay is an evaluation of a topic, this means that you are not merely reflecting on what occurred within the book but your are going to explore what the plot line meant and how you reacted to it.
If you are going to write a informative essay you will be focusing less on what the meaning of the story was but instead reiterate the timeline of events that happened within the book. Who are the main characters? What drove the plot line? What were the major events that affected the characters?