In most cases, when a fact is general knowledge, it can be stated without providing a citation. This is a skill that needs to be developed. In the context of a specific publication, what is common knowledge in one field may not be common knowledge in another. Over time, you will become better at identifying common knowledge, especially within your chosen field.
You can begin by asking yourself the following questions:
If you feel like the point you are making is likely to be disputed by a number of your readers, you should provide some sort of source to support your claim.
If you have questions about whether you should cite a fact, it is better to cite it.
You can also ask your instructor or contact a librarian for help determining whether a fact is common knowledge.