How do you do footnotes in a research paper?

I am writing a research paper for my AP US History, 11th grade cl. My teacher said that we "must cite ALL sources in accordance with either MLA or APA guidelines, papers without correct citations will NOT receive a grade over a 75" he later goes on to say that he prefers "(1) footnotes (2) endnotes or (3) social science-style in-text citations..... be consistent... there are penalties for incorrect, inconsistent or missing citations." I am only a freshman and i am very stressed out about this so please educated answers only.