gnfb
I've been where you are and so I will try to explain. What others posted above is probably very helpful as well. But, just in case you are starting with zero knowledge about Flarum, Composer, etc., here's some information.
First, there is self-hosting Flarum and there are sites like freeflarum.com. Sometimes people come here asking extension install questions but then it is uncovered they have a forum at freeflarum.com. If that's the case then you can only install the extensions they have available and I do not have experience with Free Flarum's site and so this is where my help ends with that.
If you want to self-host Flarum, some hosting providers are now offering a 1-click installation via Softaculous. This definitely makes the core installation very easy, but then you are stuck without extensions.
Currently to add extensions you have to first begin to get a little comfortable with doing things via command line (terminal, SSH, insert any other similar name here). Also, your web host must offer SSH access. Some might offer a built in UI/terminal where you can type and run the commands right from something like cPanel. If not, there's software you download such as putty.org or termius.com, connect to your web host (somewhere they give you access credentials if they offer SSH access), and then you can run Composer commands (also assuming the web host has Composer installed).
This brings us to Composer. All the extensions say something like, to install, run: composer require then the path to the extension. You type that into the terminal (or SSH client like Termius), press enter, and the extension is added.
Developers these days like this way of doing things because once you know how, it's very fast to push changes to code, install, and update extensions from a command line interface. And, Composer helps to install a ton of different packages (libraries) with a few taps of the keyboard. Personal rant: The downside is, developers have made it harder for non-developers to enjoy a lot of the new self-hosted software out there. Personal rant 2: Another upside though is that my desire to run Flarum made me learn new ways of messing with web code, which is pretty cool.
That's all I have for you for now. This was not meant to give you step by step instructions but to somewhat explain how it all works, high level. You might know this, you might not, I don't know how much you know... and so, maybe step one would be looking up information about SSH/terminal, ensure your web host offers it, and just make sure you can at least connect to it. If you can do that, then it's just a matter of targeting the right directory, typing or pasting in the right Composer command, and done.
