When completing any assignment it is imortant to always cite or give credit to the original source of information.
CSULA defines Plagerism as "the act of using ideas, words, or work of another person as if they were one's own withouth giving credit to the original sources" (Academic Honesty, Appendix D).
In many cases, your professor will tell you which style they prefer. If they don't, here's a general break down:
APA: Business, Education, Psychology, and other Social Sciences
Chicago (Author-Date System): Physical, Natural, and Social Sciences
MLA: Literature, Arts and the Humanities
Another good resource is Purdue Online Writing Lab: