When a creation is in the public domain, that means it is not under copyright and can be used freely. A creation may also be licensed by its creator to be used or adapted by others - for free! You should still cite your sources and check whether a work (image, music, web content, video, etc.) is licensed to be used or licensed to be used and adapted.
The links below will get you started with finding media that can be used for your media production projects.