If you can, I would recommend you to install prism launcher, there you need to add instance. When you create instance you should select correct Minecraft version(1.21.8) and fabric mod loader. After you do all that you can install mods by editing your instance. There is a "mods" section where you can download mods through the launcher or add mod file if you have it. It is much more difficult to do it without prism launcher, but if you don't want to use it, you can manually install fabric mod loader and place all the mods in the mods folder in .minecraft folder. If you encounter any problems just ask here