The preceding suggestions are all spot-on and should help you get started. The one strong caveat I have for you is, do NOT allow your book review to lapse into plot summary. This is the single most common error that students commit when writing book reports and book reviews.
Plot summary simply means that instead of offering your opinion and assessment of the book as a whole, you start telling the reader the story. Even if it's paraphrased in your own words, plot summary does not help you earn a good grade on this type of assignment.
When writing a book report or book review for a teacher, you can assume that the teacher either has read the book already or is familiar with the plot. Good luck on your assignment!
I assume this is more of a formal book report, so the audience is your teacher (reader) and perhaps your classmates. You've already gotten some great advice here, so let me just add an idea or two to make it a bit more interesting for you.
First, select several quotes which are indicative of the work as a whole, of one portion of the text, or of your feelings about the work. This takes a little pressure of you and will offer a glimpse of both the author's view and his or her writing style.
Also, select an interesting anecdote, explanation, or other story element to again both add interest and demonstrate specifically what a reader might expect when reading this book.
You'll want to consider the impression you received from the book, how easy it was to read and follow, if the plot made sense and was consistent (for fiction), and whether or not the book would appeal to a mass audience or just one niche of it.
For non-fiction you can assess whether or not the author is convincing, has strong source material for their claims and presents the material in a logical and straightforward fashion.
Lastly, were you bored? Inspired? Interested? Deterred? What impact did reading the story have on you?
Think first about the purpose of your book review and your audience. A book review generally gives a brief summary of the plot, shares the readers critical opinion, and typically suggests an audience who would appreciate its content (a recommendation of sorts). Nonetheless, certain components of your review must be included within your review: Title and page count, publisher information, author and author's theme(s) explored, setting of the book, and your evaluation and recommendation.
Generally, one-half to two-thirds of your book review should summarize the author's main ideas providing a brief discussion of the plot and the remainder should include your evaluation and opinions as a reader.