I mean are there courses you can take, or it's just about experience and journalism or something...
Watch a lot of movies. Read a lot of movie reviews. Try to get a gig writing movie reviews for a local newsletter or paper to get some experience, and go from there to try building on your successes.
I guess you could take general writing courses to improve your style.
I suppose there are probably courses specifically on writing movie reviews, but it seems like they might be hard to find.
It might make sense to take some film courses, although a lot of those are about larger societal issues as reflected in film -- rather than just about the quality of the movies themselves.