You can cite anything that you rely on to support an argument, counter an argument, build evidence for a conclusion you are making, for anything you represent, quote, summarize or paraphrase.
You'll want to cite whether what you cite is copyrighted or in the public domain (for example: most US government documents, material published before 1925, an unrecorded class discussion)
The scholarly conversation is ongoing. It reaches back to the earliest printed sources and continues all around you. When we write a paper or publish a book or article, we are engaging in a public exchange of ideas. Each engagement is built on previous engagements, whether we are countering a previous assertion or building on it. We are required to acknowledge it. But, it is also is to our benefit. It places us within that conversation.
Take careful notes of everything you read, view, or with which you engage
Keep your notes organized and associated with the articles, books, images, documents you are reading
Using a citation management tool or platform will help you with keeping notes and materials together and with proper formatting.