Important: Even if something is free to use, there still might be licensing restrictions you need to follow. Please ask if you have questions or doubts about exactly how you are allowed to use something.
There are best practices for providing attribution when you use materials with Creative Commons licenses. It typically looks like this:
"Title of the thing you used" by author/creator is licensed under Creative Commons license
You should also link to the actual item, the creator's user or account page (if possible), and the text of the CC license. Here is what a real example might look like:
"Tango" by Adrian Shake is licensed under CC BY 3.0
If it helps, here is a list of links to the different Creative Commons licenses: