Read the book through from cover to cover. Did you like the book? Why or why not? What do you think the book is trying to say? Can you recognize a major theme in the book? (Themes could be good vs. evil, growing up, etc.) Give some examples of plot points that illustrate that theme in the book. Who are the major characters? What are they like? When and where is the action of the book taking place? (setting)

Take notes on all these things, and then start to write.

Did your teacher give you any kind of guide sheet? If not, you might consider following this: http://homeworktips.about.com/od/writingabookreport/a/report.htm