I love movies! I grew up with movies! I am the movies!

I always wanted to review movies for the public. How do I do that?

Extremely competitive. In general, I'd say you need a college degree, and these days, probably a major in film. You need an excellent command of language at the very least, and knowledge of all sorts of literary and historical references that are in films, as well as references to older movies. Even stories like Harry Potter have deep roots in mythology. You need to know EVERYTHING. Missed that quote from Cicero, or Shakespeare, or Ray Bradbury? You are toast.
Then there's direction, cinematography, geography and settings, and continuity. There are people who love to find continuity faults, and you have to find them too.
And so on. Good luck. If you want the job, you'll get it.
Start with a local paper. Offer to do them for free.
Or start here. Critique Gravity and we will critique you.

Keep in mind that all journalism has changed radically. Sites like Rotten Tomatoes are king in your field, with the audience providing the reviews, all for free. Way back 25-30 years before you were even born, my hero movie critic was Pauline Kael.

First, I would start your own blog (I suggest Wordpress) where you write reviews of movies. Then send local newspapers and other media links to your reviews. Try to get a following with good comments about your reviews.

As Joy said this is a competitive field. But with the Internet, anyone can start a blog and post reviews. The question is whether you can get a big enough following and impress editors who might want to give you a job.

I agree, what is your education, consider a degree in film and media. Many start with publications in lower level jobs and work for years to get positions.

I agree, start with online blogs and see what happens. If you can get enough readers and followers, you get advertising and make money.

Since you are 15, I suggest starting with seeing if the school newspaper would be willing to give you a chance to review movies for them.