Go to movie reviewing forums and create a profile. Start typing some of your favorite movies in the "search" box and read what people think of them. You might agree or disagree with some of them but that is just the start. You want to get a feel of how people like to read or write about movies they have seen. Most importantly, though, just browse and observe. Don't give your own opinion for at least two weeks. Is there a genre that people aren't reviewing? Are the reviews very serious and technical or are they just fun and loose? Whatever it isn't is where you want to be.
Throw your two cents in after just reading the forums for two weeks. Try to post in a way that is opposite of how everyone else is posting. For example, if people are focusing on the technical aspects of a movie like "Wizard of Oz", start a topic about the funny parts in the movie or the most emotional scene. This will get people thinking outside of what they are used to seeing and becoming interested in who you are.
Start up a free blog. There are numerous sites online that give out free blogspace. Pick one with a name that is personal but obviously refers to movies. You'll have to be a little creative, but the easier the name is the quicker people will remember it and go to your site. Be sure to include a counter so you can keep track of how many people are going to your site. Develop a catchy rating system, usually going from "never seeing it again" to "instant classic", or 0 to 5.
Start putting together a few movie reviews of your own on your blog. Just take a few classics that you love or a genre that the forum isn't reviewing a lot of. Once you get about five reviews, make them open for commenting. Add the title of your blog to the signature block of your forum profile.
Continue to post in various places of the forum so that people get to know your online name. Answer a few questions that people have about movies that you can answer. Even put in a funny post or two. Keep this up for a couple of weeks. People may start asking about your blog or even visit it without you outright advertising it.
Begin to outwardly advertise your online movie blog. By waiting those weeks and posting you avoid being seen as a "spammer".You are a legitimate online movie critic with opinions and a rating system. Start out by sending a private message to people whom you've had online discussions with. And if the forum has a place for it, post a topic about your blog and what makes you different from other critics.
Add your blog to the signature page of every e-mail or forum you post in. This will help continue the buzz. A critic is nothing without an audience, so you need to create one. Make simple business cards and give them to people you know that love movies. Be sure to give one to the manager of your local movie theatre. If you throw out a shout-out or two about the theatre in your review, who knows? He may give you access to some advanced screenings or even free tickets!