An Annotated Bibliography starts with a traditional references list (a.k.a. "Works Cited" or "References" if accompanying a research paper). Listed sources might include journal articles, books, primary sources such as photographs or personal interviews, reliable websites, government documents, or any other resource you might use as a basis of a college-level research paper or published document.
The "annotated" aspect is your explanation of the content of each entry, highlighting it's value/contribution to your research topic. (Do NOT merely copy the abstract or article summary from a research database or even the article itself. The annotations should be in your own words.)
Determine if you need a primarily descriptive annotation, or an evaluative one (often called "critical" annotation) that includes your perspective on the value or effectiveness of each entry. If you are building a list of resources to refer back to for future projects, your annotations should include both the descriptive and evaluative aspects.
How much detail? The content (scope and length) of an annotation changes with circumstances.
Course assignment: Check your syllabus for instructions, or ask your instructor for a sample "good" annotation to use as a template. You might mention how the entry relates to course readings - does it reflect or contradict other research? Is it appropriate to use as a resource for a college-level research paper?
Publication: Check the "Instructions to Authors" and/or review recently published entries within the same journal in which you hope to publish.
Personal reference: In addition to content summary, you might include unique characteristics such as research methodology or survey tool, illustrations, maps or other graphics, statistics or datasets included, etc.
Bibliographic citation following assigned style (e.g., APA or MLA).
Plus, any of the following:
Descriptive or Factual Content
Evaluative or Critical content