Take this free online course: https://ocw.mit.edu/courses/21l-011-the-film-experience-fall-2013/

Watch as many films as you can, and over time your understanding will improve as you become more film literate. When I did this, I got a lot of the films from my local library. I augmented each lecture by watching more than the two films assigned for each lecture. I immersed myself in film history, and soaked it all up, and it's still ongoing, although I've seen over 2,000 films since I started with the course.

You may come to find that you don't always agree with the consensus whether or not a film is 'great.' That's ok, in fact, that's good. Critical thinking is what's needed.

Film is a subjective medium, and everyone sees films through their own mind's eye and their own things that they bring with them. It's ok, and preferable to form your own opinions. As you get more film literate, you'll find it less important what others think of a film. All that matters is what it means to you.