Citations are the way scholars help other scholars do research. It's also a way to provide evidence for the assertions you make in your research. Using just the citation someone reading your paper should be able to locate the source you used. This is especially important for historians, who have everything ever produced in the past to work with. Your citation should help someone reading your paper 25 years from now locate the material you are citing.
Many databases include links to help with your citations. Each database is different, but search the page for a link that provides pre-formatted citations for the article you are reading.
The Purdue Online Writing Lab (OWL) is an invaluable resource for crafting your own citations. Inevitably you will have materials you want to cite that you did not find pre-formatted citation information for. If you need some guidance on how to format your citation, either in the refererence or bibliography at the end of your paper, or in in-paper citations, visit the OWL at Purdue for assistance.
WorldCat also provides citation information for books and articles. Visit the WorldCat page, type in the title of the book or article you want to cite, select the proper result and you will see a Cite/Export link just above the record on the right-hand side.
Many databases can also export to Endnote and other citation software.