Each entry in an annotated bibliography has two parts: 1. the citation and 2. the annotation.
The citation should be formatted according to the guidelines of the citation style your instructor has asked you to use.
The annotation is not a mere summary of the source; it is also an evaluation of the source. It should briefly summarize the work cited, then include several sentences evaluating the source and, often, explaining how you will use it. You can do this by evaluating the authority of the author, explain how you will use the source in your research (examples: source for data, use works cited for further source exploration, primarily for background information, etc), compare or contrast it to another you are citing, explain how it illuminates your topic, etc.
Some questions to focus on in your annotation:
What information does this source contribute to your research question?
How does the source relate to other sources in your bibliography?
Does the source appear to be biased? If so, how is it biased?
If it's an article, what are the strengths and weaknesses of the article?
How does the source fit into your research paper? Is it useful? Why is it useful?
Consult the Cornell University Library's How to Prepare an Annotated Bibliography page for more details and examples of annotated bibliography entries.