National History Day prefers either Chicago or MLA style citations. Noodle Tool, which will format citations into MLA, is partnering with National History Day. There is a special page on this at the NHD site. Detailed information on how to prepare the NHD annotated bibliography can be found on the NHD site.
For general information on formatting citations, Purdue's OWL site provides examples of several citation styles. The Chicago Manual of Style offers some examples on their website.
Many databases, including Google Scholar, will format a bibligraphy entry for a specific items, and offer a variety of citation styles. If Chicago is not available, use Turabian, which is very similar.