Place a footnote at the bottom of the current page, as here.
Noël Coward quipped that having to read a footnote is like having
to go downstairs to answer the door, while in the midst of making
love.\footnote{%
I wouldn't know, I don't read footnotes.}
You can put multiple footnotes on a page. If the footnote text becomes too long then it will flow to the next page.
You can also produce footnotes by combining the \footnotemark and
the \footnotetext commands, which is useful in special
circumstances.
To make bibliographic references come out as footnotes you need to include a bibliographic style with that behavior (see Using BibTeX).