Use Chicago Style, and stick to one "flavor" of Chicago
Chicago is the discipline standard citation style for historians, and should generally be your first choice when writing history papers. While Chicago has several different variants to choose from (author-date, footnote citations, etc.), it is best not to mix them, to keep the styling of your paper clear and consistent.
Use a Citation Manager
Citation management software allows you to quickly collect citation information for interesting items you find online, while also allowing you to track and organize this information long-term. Additionally, citation managers can automatically generate citations in the style of you choice, saving you the hassle of writing them yourself. See more info below.
The University Library recommends using Zotero as your citation manager, due to its open-source nature, consistent performance, and robust feature-set.
The current calendar of upcoming Savvy Researcher Workshops, which offer training and assistance for citation managers, as well as other general and specialized research skills.
Purdue Online Writing Lab's introduction and style guide to annotated bibliographies. Chicago style annotations are covered towards the bottom of the page.