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. See the image at the bottom of the page for a quick overview of how this works.
The links below will get you started with finding media that can be used for your media production projects. This is not a comprehensive list.