Reason
It's not only specialists (e.g. journalists, critics) that write reviews on books or films. More and more reviews are written by laymen, as you can see for example at the online bookstore Amazon.
Preparation
Recall the plot of the story. Do you want to recommend the book/film?
Structure and Content
- Introduction, Background
Give title and author of the book. If you're reviewing a film you may want to mention the director or actors. - Plot
Summarize the plot in a few sentences. Say something about place and time of the plot, about the main characters and about the main content. - Comment
Let others know whether or not you liked the book/film and give reasons for your liking / not liking it. Is the author's style good or bad, is the book / film interesting or boring etc.
Important Tenses
Note
A review is not a summary. Don't tell the end as this will spoil the book/film to people who want to read/watch it.