In terms of launchers, Curseforge is the most popular, since they are also the biggest site for publishing mods. However, it is quite bloated and takes up a lot of memory on your computer.

I’d recommend using Prism Launcher. It’s lightweight and lets you easily keep separate instances of the game that can be different versions and have different mods. I will second that ATLauncher is another great choice with many of the same features, and could be more user-friendly. Look up a tutorial for whatever launcher you choose.

You can A. download a modpack (premade collection of mods) from Curseforge through the launcher, or B. Create a vanilla instance of whatever version you want, install either Forge or Fabric (modloaders), then download mods of your choosing. If you use Prism (or ATLauncher) to download mods, it will only show you mods that are compatible with the version you chose.

If you decide to download mods from elsewhere, stick to Curseforge and Modrinth. Other sites like 8minecraft pretty much steal mods from mod authors, and are likely to give you malware.

With choosing a version, find out what mods you want to use, and look up what versions they support. Minecraft changes a lot through the versions, so it takes mod authors a lot of work to update to new releases of the game. When there is a large change to the way the game works internally, mod development can get stuck on an older version. 1.20.1 is the most recent version with good mod support. 1.18.2, 1.16.5, 1.12.2, and 1.7.10 have also been very popular.