Use the "Chicago Manual of Style" link to the left to connect their clear guidelines for both humanities and social science citation. You can use whichever style you prefer, and that makes disciplinary sense, but strive to use the style you have chosen consistently.
Here are some general citation rules not discussed in those guidelines:
The superscript number for notes should go at the end of your sentence, after the period, and if the sentence ends with quotation marks after those too. Use Arabic numerals for superscripts; Roman Numerals can get quite unwieldy once you go beyond v.
If you have more than one source to site in the same sentence, put the information into a single note in the order that it occurs in your sentence. If you have many sentence in the same paragraph that draw material from a single source (even if it comes from different pages) you can combine the information into a single note, again putting the page numbers in the order that you use them. In either situation if your material seems confusing, it is always fine to include identifying phrases, eg. for information on Chinese immigration see John Tchen, New York Before Chinatown (Baltimore: Johns Hopkins University Press, 1999); for a contemporary account of tenement fires see Jacob Riis, How the Other Half Lives (Boston: Bedford Books, 1996), 37....
The second time that you refer to a source, you don't need to repeat all of the citation information. Instead just give the author's last name, A Short Version of the Title, page number. You may have been taught to use ibid and op. cit. for this situation, but contemporary practice really favors this form (for good reason, readers don't want to have to look back through earlier notes to figure out what your source is).
There are as yet no standards for citing the census from these new modes of accessing it. So be grateful that you are not using microfilm:
For the historical census give us Census year, place, ward number, page number, name and address.
For the contemporary census give as much information as a reader would need to replicate these results for e.g., reference Census 2000, data file 3, Columbia County, Maryland, aggregate totals of xyz and abc.
For periodical articles found through electronic data bases you can simply cite them as you would the same source found in a paper version of the journal. For information found on the web itself give the full URL.