Where can I find mods for Fallout Four?

The Nexus collection of sites is a group of sites that most people use when they are looking for mods for games, especially Bethesda games. The Fallout 4 Nexus has quite a number of mods already, ranging from simple ones (like removing or changing a single sound effect) to larger and more complex mods (like a few that drastically increase the number of objects available when building your settlements), so you might want to start there.

There is a premium membership to the Nexus sites, but you can also sign up for free and get all of the mods you want.