When do you need to cite your sources? The short answer is that you should cite a source anytime you incorporate an idea, quote (written or spoken), data, image or other content that is not yours unless it is common knowledge.
The term common knowledge refers to any knowledge that you can reasonably expect other people to know. For instance, the fact that there are bilingual speakers in the United State is common knowledge. You would not have to cite any sources.
The specific percentages or numbers of bilingual speakers would not be common knowledge. If you were using any graphs or numbers about how many bilingual speakers are in the United States, you would need to cite where you obtained the information. If someone, such as a professor, told you in person or via email, you can cite it as a “personal communication.”
There are many sources of research information available, and several citation standards such as MLA, APA, Chicago Manual of Style and more. However, you should always follow whichever citation or bibliography format your instructor gives you.