For an assignment (such as a research paper), try the turabian-formatting package. It provides "Chicago-style formatting based on Kate L. Turabian's 'A Manual for Writers of Research Papers, Theses, and Dissertations: Chicago Style for Students and Researchers' (8th edition)."
The turabian-researchpaper document class, included as part of the turabian-formatting package, will produce a research paper that has "CMS formatting" and is "double spaced, in 12pt font."
Example LaTeX code that implements the turabian-researchpaper document class:
\documentclass{turabian-researchpaper}
\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage{csquotes}
\usepackage{biblatex-chicago}
\addbibresource{my_bibliography.bib}
\title{The Title of Your Work}
\author{Your Name}
\date{Date}
\begin{document}
\maketitle
The content of your paper.
\printbibliography
\end{document}
To produce appropriately-formatted citations, you will also want to use the biblatex-chicago package (as also noted above).
biblatex-chicagois a very rigorous implementation of CMS guidelines. Where does it fail for you? (Also, if there is a real bug, the author of the package [not me] would like it reported!) – jon Nov 24 '11 at 18:20