"Do I have to cite this?"
When in doubt, the best course of action is to provide full citation.
As you research, you will encounter a lot of different types of information and sources. Any information that was created, discovered, researched, and written by another person always has to be cited. This includes, but is not limited to:
You do not have to cite the source of every fact and figure in your essay or speech. In particular, you do not have to cite information that is considered "common knowledge." What is considered common knowledge is dependent on your audience and is context specific.
Common knowledge is a fact, figure, or quote that is widely-known or easily verified. Some examples of common knowledge are:
Cats are mammals.
George Washington was the 1st President of the United States.
The Chicago White Sox play at Guaranteed Rate Field.
The United States of America has a population of about 320,000,000.
These four statements can be considered common knowledge. If a reader wants to verify that “Cats are mammals,” most books about cats will confirm the fact.
It is very important to understand that what is considered common knowledge is context specific. Facts and figures about the United States may only be considered common knowledge for people in the United States. For example, how many people in the United States know that José María Castro Madriz was the 1st President of Costa Rica? It may not be necessary to cite the fact, basic information about country leaders can easily be verified, but, in some contexts, it may help your audience by pointing them to a source.
Common expressions do not have to be cited. These are widely used idioms that appear in day-to-day speech. As part of the common lexicon, it is also may not be necessary to add quotations around these expressions, as you would have to do with other direct quotes. However, you must be careful that you are not trying to claim the expression as your own, original words.
Examples of common expressions: