The kind of principle described previously works well for fiction books and most Hollywood movies. However, if you are reviewing a non-fiction book in some specialized field, or a documentary or avant-garde film, then you may not have enough experience with the genre to know ahead of time what your principles are.
One way to start is to figure out what purpose the author or filmmaker has and whether or not she achieves that purpose. In a non-conventional film, the director's purpose may be difficult to determine, and a great deal of critical thinking on your part may be necessary. You might arrive at the conclusion that the filmmaker's purpose is to leave the viewer with a sense of confusion. If you can support your perspective with evidence, then you have something to write about.
One way to articulate this position might be to write, I do not care for confusing films; however, the filmmaker has accomplished her purpose. On the other hand, non-fiction books often contain a preface in which the author states her purpose. If your author’s purpose (as indicated in her preface) is to educate the reader about bipolar depression, and the author spends most of the book talking about unipolar depression, then the author has not met her goal.
Gauging how well the creator has achieved her purpose is the best way to write a fair review, even if you hate the book or movie you are reviewing. You have the right to the principle I don’t like books about psychology or I don't like avant-garde films, but if your assignment is to write a psychology book review or a critique of an avant-garde film, then the I-don’t-like principle won’t get you very far. If you say instead, I can respect books or movies in which the creator clearly states her purpose and then accomplishes it, even if the subject matter does not interest me, then you have somewhere to go with your review.
Next, learn how to take notes.