how to critique a movie

In this topic I want to ask you how to critique a movieThis task I got from my teacher, and I don't know how to do it right. At first, I need to choose movie. I think, it is not so hard, because there are many good films, which I saw, and I have own opinion about them. But, what I should to do next ? I can't write bad things, because every film it is a someone's art and many efforts.