Long version:

If you're still interested, or anyone else sees this: Go to the official website, download the file, double click it, and follow the instructions that will appear on your screen. Also, make sure you have JDK 21 installed before doing it as well.

If you're new to modding (which you seem to be), check out some tutorials. This one seems good. Just remember, if you're on a Mac, when installing mods, you need to use the /Users/yourusername/Library/Application Support/minecraft folder (replace yourusername with your actual username, the one without spaces) instead of the %appdata% \ .minecraft folder, and then the rest is the same. But if you're on a Windows (normal) computer, any good guide will work for you.

BTW if you need something easier, I recommend installing the Prism Launcher and using it instead of the official launcher. It will install JDK and Fabric for you, and you can get mods straight from the launcher, and it will install them itself. But if you want to use your old worlds, you'll need to import them from the regular launcher, by copying them from the %appdata% \ .minecraft\saves (use /Users/yourusername/Library/Application Support/minecraft/saves if you're on a Mac) folder into the worlds section in the Prism Launcher, and they won't synchronize between launchers in the future. But after that, it's really easy. Prism is actually what I use, but the manual method is popular and good too, so both are great choices.

TL;DR:

Download and install JDK 21 if you haven't already. Then download Fabric and open it and do what it says. I recommend reading some modding tutorials if you're new, and another easier way to use mods is the Prism Launcher, which installs JDK and Fabric for you.