It depends on how serious a reviewer you want to be.
Do you want to make a career of being a reviewer or do you simply wish to review movies that you have seen as basically a hobby,
If its the former you will need to prove to a newspaper or movie magazine editor that you can create and write a subjective review of movies you have seen and hope that they are happy with it.
As far as what you need to be a reviewer. Really nothing except a pen, paper or computer and printer. You also need an excellent command of the English language.
If you simply wish to review movies as a ‘hobby’ there are many online sites you can subscribe to and then get started on writing reviews of the movies you have seen.
The majority of these are free to join and you have a free hand to write your reviews.
The site I subscribe to is IMDb (Internet Movie Database). It is free to join and you can write as many reviews as you want.
Always remember though a review of a movie, or indeed a review of anything is only the personal views of the person writing that review. and it is unlikely be shared by everyone.
I read peoples reviews and some think what they reviewed was wonderful while others simply think it was terrible but as I said before it is someone else’s personal feelings.
Personally I read many movie reviews but I make up my own mind what I personally think of the movies when I have watched it.