How do you do footnotes?

Every time I had to do a reseach paper I always did it in MLA format with intext citations and a bibliography at the end. For the first time I was igned to do one in APA with footnotes. Can anyone explain what footnotes are extactly and how do you do them. If I do footnotes, do I have to do intext citations as well. I am only using books, so how would I put that in a footnote. Any help would be greatly appreciated. Thanks