First, a disclaimer: I don't perceive myself to be the grand master of writing movie reviews, so you shouldn't consider me as such. But I've written quite a number of them, and though I don't feel like I'm the best writer out there, I certainly think I'm better than some people I've come across online. (And a lot of them are simply bad writers)
With that in mind, I've come up with some tips on how to properly write a review on a film you've watched. It's just a list of stuff, I'm not going to tell you how to structure your little opinion, but I will tell you what to do or not to do, and some things you'll need.
1. Have a good command of the language you're using
If you're going to write in English, then you better damn well have good command of it. I don't expect you to write like a professor, but you should have good grammar at least. I've come across reviews that were obviously written by people who don't speak English in their everyday lives. I don't mean to pigeonhole or be a racist or anything, but some of these people are usually referred to as 'Ah Beng' or 'Ah Lian' based on their grammar skills. If THAT'S how you're going to write, better for you to write in your mother tongue. (I know your teacher probably told you that practising your writing will help you improve, but if you're going to put your writing on display to the public, best not to do so)
2. Don't post so many pictures
Many online reviewers like to post pictures of the films in between paragraphs of their article. Two things: One, these pictures were most likely already posted by the film's marketing team earlier, so you're just showing what most of your readers have already seen. Two, your pictures are just making your article seem longer, and if your article isn't that long to begin with, it's going to look obvious that you're not that good a writer. Stick to two pictures per writing, and not more.
3. Use your own words
A lot of reviewers start their article with a synopsis i.e. a description of the film's plot in a nutshell. Most of these reviewers just copy it from the film's website or from Imdb. Again, two things: One, not using your own words makes you unoriginal and lacking creativity. Two, it makes you look lazy.
4. Don't post the film's trailer in your article
You know what's the best thing about the internet? You can find anything. Using Google and Youtube, anyone can find a trailer about any film they want to know about. So why do it for them? Again, this is yet another attempt to make your article look longer than it really is.
5. Be honest
This is quite hard to do actually. Even I had problems with this for the first few years I wrote reviews. There were times when I didn't have the heart to say that I was disappointed in a film I had high expectations for. Over the years, I got better and now I'm less afraid to speak my mind in my writings. Bottom line is, don't be afraid to say what you think.
6. Keep it real
Using humor to make your review more interesting is okay. A funny review is better than a boring one. But tone down the jokes. I've seen people make Instagrams and memes to make fun of the film they're talking about, and to me, that's too much. Even if you don't like the film, you should have some respect for it. Throwing a joke or two by referencing pop culture is fine, just don't overdo it.
7. Get your facts right
Somewhere along the way, you'll want to reference information in talking about your movie. It's vital that you get your facts straight. Make sure you know them and state them correctly, if you're uncertain, look it up. Imdb and Wikipedia are good sources of information to go to.
8. Use a rating system
This is arguably the best way to tell your readers if a movie is good or bad. It will also be useful for certain lazy people who don't want to read your supposedly lengthy article. Either use a 5 star rating or a 10 star rating to tell them where you stand.
9. No useless information please
There was this guy who posted his review on Facebook, in a movie review group no less. I read it, and it turned out to be a personal blog entry on how he spent his day, from how he met his friends and decided to go watch that movie, to meeting a girl along the way etc. And when he finally got to the part where he had to talk about the movie, he gave two sentences and a rating. And that was it. Aaaannnnnnnnddddddd this guy wants to post this to a movie review group's wall? Seriously? I bet you can tell that I don't give a damn about his extracurricular activities, and he just wasted my time. Guys, please stick to the point at all times.
10. Don't be biased
As far as possible, don't walk into a film with a pre-decided opinion on its quality due to who's directing or starring in it, or what genre the film is. If you do, you probably won't give it a fair shake. This is why I usually stay away from films that I know deep down I won't enjoy. Over the years I've developed a way to be as fair as possible to all films I watch, even if it was bad. This is reflected by my ratings, which is rarely extremely leaning in one direction.
Lastly, a piece of advice. Something every reviewer must have. Passion. All writers must have passion in their work, no matter what their subject is. So you, as a reviewer, must have passion to do what you do. I know what it's like to not want to write my opinion on a movie even when I have it in my head. It's the timing, or the mood, or something else. I have missed out on a handful of opinions in the past, something which I intend to get back to and write about at some point. The point is, if you have the will and passion to put your opinion out there, then do it. If you don't think you can do this over and over again, then don't do it.
And that's about all I can share. I'll still say I'm no expert, so don't take my words as law. But this is what I think you should do, and if you do it right, you'll be great in my book. Or better perhaps.