There are different types of creative commons licenses that dictate what you can and cannot do with the content you find online. Some CC licenses allow you to download content and share it with others, and some licenses may let you change or edit a work. Most licenses will still require that you cite the original source. Be sure to read up on the different licenses and what each one means. This is an example of a CC license that you may see attached to someone's image, song, etc.
Check out some of these real Public Service Announcements to get ideas for your project.