For a list of Fabric mods, you can go to the mod-hosting sites CurseForge or Modrinth.
To add a mod to your Fabric client, navigate to the mods folder inside your Minecraft directory. If it does not exist, create it now. Download the JAR of the mod you want, and drop it into this folder. Most mods will require Fabric API; you can place the JAR for the API in /mods as well.
To add a mod to your Fabric server, navigate to the mods folder inside your server installation. If it does not exist, create it now. Download the JAR of the mod you want, and drop it into this folder. Most mods will require Fabric API; you can place the JAR for the API in /mods as well.