To use mods on Minecraft Java Edition you need a mod loader (Fabric, Forge or NeoForge) for your exact game version. Then you put the mod's .jar file in the mods folder and start the game with the loader's profile. A launcher like Prism or the Modrinth App can do all of this for you.
Every mod page says which loader it is made for. You can't mix them: a Fabric mod doesn't run on Forge.
Start the official launcher once: a new profile with the loader's name appears under Installations.
Download the .jar file for your Minecraft version and loader, then copy it into the mods folder:
%appdata%\.minecraft and open the mods folder (make it if it isn't there).~/Library/Application Support/minecraft/mods~/.minecraft/modsDon't unzip the .jar. Start the game with the loader's profile; on Fabric and NeoForge a Mods button on the title screen lists what loaded.
A modpack is a ready set of mods that work together. Don't copy its mods by hand: open the .mrpack file with the Modrinth App or Prism Launcher, or install the pack from the CurseForge app. It makes a new instance with the right version, loader and settings. Give it 4 to 8 GB of memory in the instance settings.
Mods that only change your own game, like minimaps or performance mods, work on any server. Mods that add blocks, items or mobs need the same mods on the server. The modded servers list shows what each one needs, and how to use mods on a Minecraft server explains the rest.
Browse thousands of mods in our resources, each with its versions and loader listed.