Common knowledge does not need to be cited. Common knowledge includes facts that are known by a lot of people and can be found in many sources. For example, you do not need to cite the following:
Your own work that has not been previously published or submitted elsewhere does not need to be cited. In general, if it's your words, your opinion, your photo, or your graph, of course, you don't need to cite it. HOWEVER, if you are using information from one of your own previously published works (journal article, book chapter, etc.), you MUST cite it just as you would cite another author's work. In addition, if you have previously submitted a paper in one class, you can not submit it in another class as that would be considered a form of cheating.
Photo from https://commons.wikimedia.org/wiki/File:Abraham_Lincoln_November_1863.jpg