Aspiring critics should always observe movies based on their implied context. They can't, for instance, criticize a summer action film for not taking itself seriously, or a Holocaust drama for not being funny. Gauge a movie's general intentions -- whether it wants to push creative boundaries, impart a social message or simply entertain the audience -- as a basis for critique.
One person may believe a film succeeds at its task, while another may strongly disagree (Critic Gene Siskel and Roger Ebert built an entire media empire based on such disagreements). But once you have an idea of what a movie sets out to accomplish, you can gauge how successful it is at its task. When answering this question, cite specific evidence in the film to support your conclusion. Don't limit yourself to vague generalities; look for performances, dialogue, action and particular narrative elements that either make the film a success or sink it. Refer to such evidence in your review if you want your opinion to carry any weight.
Most films follow a given formula and/or borrow elements from older films that have come before them. That doesn't make them failures, but falling into a narrative rut challenges their ability to stand out. A film that flashes some genuine imagination -- subverting an existing stereotype, avoiding story clichés or delivering a character or set piece that you've never seen before -- is more distinctive than other films.
The best way to gauge your opinion of a film is to ask yourself if you want to see it again. If you do, it's likely done its job -- either because it contains material you'd like to study further or because you simply enjoyed yourself while watching it. If you don't, that doesn't necessarily make it a bad film (especially if the content was serious or intense), but you need to cite its positive aspects (strong performances, for instance, or a challenging message) deliberately and specifically before recommending it. You then need to cite the reasons for wanting or not wanting to see it again and convey them to your reader in an engaging fashion.