First and foremost, a book report must tell why a book was interesting (or was not interesting!) and certain substantive information must be included in the report. That being said, there are three basic approaches you can take to a book report. You can tell about the themes of the book, the characters of the cook, or the plot summary of the book. Does your assignment specify one of these approaches? If so, you will, of course, follow that approach. The specifics of each approach are ordered according to correct formatting, if you just follow step by step.
First, if you are going to write a report focus on the theme(s) of the book (theme is the large idea that the author wants to teach the reader), make a clear statement of what theme you will talk about. Use lots of quotations from the text to show that the theme is an important part of what the author wants to say in the book and explain how the quote defines the theme (e.g., Oliver's statement "Please, Sir. I want more" proves that one of Dickens' aims was to expose the basic need in all people for more than subsistence.). Since a book report is about your opinion, say how this theme affected you and whether the theme made the book enjoyable for you (or not!).
Second, if you choose to write about characters, you'll look at physical, personality and psychological descriptions of the one character you choose to write about. Describe the character's physical appearance from hair to vest to shoes and backpack. Then say what personality traits the character has: fun-loving, courageous, quiet, athletic, studious, etc. Next mention psychological qualities of the character. Does the character have a particular flaw, like hypersensitive or rebellious or out of control habits or claustrophobia or a shop lifting habit, etc.? Next, present sections of dialogue to show how s/he talks, whether with slang or vulgarities, with elegance, with business jargon, like a college professor, etc. Then, explain how the character advances the plot by encountering, overcoming, resolving or succumbing to conflicts and tell how s/he added to your enjoyment or otherwise of the book.
For a plot summary book report, give a summary of the story, conflicts, complications, climax, resolution but go further than that and analyze the book. Tell your opinion of what makes the book good or bad, successful or unsuccessful, etc: why is it interesting, breathtaking, heartbreaking, true-to-life, creepy, disgusting, etc. use quotations from the story to exemplify the states you make about your opinions of the book.
The things that must be any book report are: the kind of report you are writing (character, plot, theme); the book title; the book author; the time of the story (World War II, 12th century, 23rd century, yesterday, etc.); the location of the story; the names of characters that you talk about (leave out ones you don't need to bring up in your report) and add a really short (one sentence or two) description of each. Also every book report must have lots of quotations and examples to show what you mean when you state your opinions.
[If you want more help on book reports, TeacherVision.com has a very good article about book reports with helpful suggestions.]