Find an outlet for your work. Either find a blog that will accept movie reviews or start your own. You can create an account on a blogging site or purchase a domain name and start your own site from scratch. Starting a blogging account from a known site is probably easiest.
Find a movie you want to review. When you attend, make sure you are not tired or otherwise unable to fully focus on the film. If you want, take some notes about character names, well-acted moments and other details.
Sit down at your computer and get ready to blog. State your opinion of the movie in its entirety. Note what was good, what was bad, how the actors portrayed the characters, if the script was imbecilic--these are all points readers would like to read about. This is your personal experience of the film, so incorporating personal pronouns like "I" and "me" is just fine. Be sure to include character names and the names of the actors who played them. Check the spelling of both so your blog is as accurate as possible. No one will take your opinion seriously if a famous actor's or director's name is incorrect.
Once you've written your review, read it over and edit it. Look for mistakes in content, grammar and spelling, and make sure it flows in a natural way.
After thoroughly editing, publish your content and await any reader comments.