You may think Writing a Movie Review only takes being able to articulate your opinions cogently and clearly on a film but there are a few things that go into it that will keep the reader coming back for more.
Have a distinct voice
Infusing your own unique voice within your movie review is essential for standing apart from all the other film critics in the market.
This should go without saying but I have fallen victim multiple times to zealous movie review writing without properly editing the draft for spelling errors and clean sentence structure. This happened (a continues to happen, damnit!) because I wanted to get my opinion out there as quickly as possible. What you should do is write the movie review, leave it for a time then come back and read it with fresh eyes. You will be able to pick out mistakes and errors you did not even know were present.
Analysis what you have seen on screen, do not simply recite the events of the film
This is lazily writing. Dive deep into the granular underpinnings of the film. It will make your movie review far more compelling and interesting to read. Here is a good example of this: Film Review for Star Trek (2009).
Do not recite the synopsis of the film in your movie review
This is a maneuver I have seen countless times to lengthen a Movie Review. Do not fall into this trap. People reading your review most-likely already know the main idea of the story. You do not need to recite it for them again. A movie review is not a book report. It is your opinion on a film you just watched so give your opinion. If your readers want to read a film synopsis, they can go to IMDB.
Argue your point of view on the film adamantly. Back up your arguments with factual information and proof that your point of view is a valid one. Reference other movie reviews or articles you have written to bolster your position on the film. This will also serve to keep people on your site longer and give them other analogous written material to read.
Have a strong conclusion
Some film critics, like Roger Ebert, end their movie reviews organically but for newbies, structure will do you some good. A summation of your opinion at the end of your movie review, like with a term paper, is your best bet. Since you already know how to do that from English class (at least I hope you do), that should be your structural start point for the conclusion of your movie review.