People often assume that writing a review of something is a very simple process. You watch/read/attend the subject in question, and then you give your opinion about it, right? There's a lot more to it, though, if you want to do it right. The steps aren't particularly hard to follow, but they do require some thought.
I started writing reviews of restaurants, theater performances, and various arts and culture events back in college. I even won an award for my column during my senior year, but the best part was getting free tickets and press passes to events that I could never afford to attend on my own dime. Now I write mostly film reviews for Examiner.com as the National Classic Movies Examiner. Over the years, I have also had to teach review writing to college composition students. At any rate, I've got some experience with the process, and you can probably trust my advice.
First, you need to watch/read/attend the subject in question, but you also need to develop a broader sense of the category to which it belongs. Go to a lot of plays, see a lot of movies, eat at a lot of restaurants before you start reviewing them. Get a sense of the range and the history of your subject. You are more credible as a reviewer and will be able to offer a more balanced opinion.
Second, do a little research to see what other people have to say. I generally read IMDB, look for reviews from respected critics, and spend at least an hour brushing up on material relating to the film before I write. For film reviews, the bonus features and commentaries on DVDs and Blu-rays can be a great resource. Build up a personal library of books and go-to websites that you will rely on when you write. You will not want to repeat what these sources say, but you will want to know what is generally thought and be able to establish your own review as something distinctive but well-informed.
Third, start writing. You review should provide some basic information about the item in question, but it should have a clear structure that targets a few key aspects that you want to emphasize. Do not attempt to exhaust the subject. Pick two or three elements - either good or bad - and explain their effect on the overall experience. For a restaurant these might be service, ambience, and the food itself. For a film, these might be a particular performance, a theme, or a technical aspect. For films, plays, and books, be sure to keep plot summary to a minimum. NEVER tell the whole story! Avoid spoilers as much as possible. All too often I read film "reviews" that are really just extended plot synopses. Make sure that your review has a clear structure with well-defined paragraphs. It ought to go without saying that the review should be grammatically correct and free from any distracting technical problems with spacing, punctuation, font, etc.
Finally, read what you have written! Good proofreading skills are invaluable to writers of every kind. Especially if your work will appear in print or online, you want it to represent your best effort.
Good luck with your review!