Below are links to guides for the three major styles used for most academic papers or research in the humanities, social sciences, and some scientific disciplines:
There are a few free online tools to help you manage your citations across projects:
"Ethics, copyright laws, and courtesy to readers require authors to identify the sources of direct quotations and of any facts or opinions not generally known or easily checked."
Chicago Manual of Style, 15th edition (Chicago: Chicago Univ. Press), p. 594.