Q. Can I cite myself (or just not reference anything) if I have a paper to write, and I know everything about the topic?
If what you are incorporating into your writing is simply common knowledge, you will not need to cite the source of this information.
However, if the information you are incorporating into your writing is more complex or "insider" knowledge, then you will need to cite current, credible sources that prove your position. Being a professional implies that you are aware that there are things you do not know and that you make a practice of checking yourself and your assumptions and knowledge. Your resources are your evidence that the information you are including in your work is indeed true.
Instead of simply providing your own knowledge with no references and citations, you will want to double check the things you know by finding reputable, high-quality resources (books, articles, credible websites) that support your point of view and/or knowledge. Cite those resources as back up to what you know. For class assignments, you may want to look at your textbook, lectures, and course readings, or you may want to do extra research in the library's databases.
Also, know that general facts known within one field or discipline may not necessarily translate as general knowledge to another. You may think something is obvious (when it is not) because of your professional experiences. A good rule of thumb would be to acknowledge ideas that are not common knowledge among your peers (other students in the course or the instructor if they are in a different field from your own).
Students should also be wary of using a research paper written in one course in another course without permission from the instructor. According to the APA Manual, 7th edition, "...incorporating previous classwork into one's thesis...and building on one's own existing writing may be permissible; students who wish to do this should discuss their ideas with their instructor..." (American Psychological Association, 2020, p. 256). Students should consult the Academic Integrity Policy to ensure compliance.
American Psychological Association. (2020). Publication manual of the American Psychological Association (7th ed.). https://doi.org/10.1037/0000165-000