Among other things, the purpose of an annotated bibliography is to help you articulate what are the main debates and the state of research in the scholarship on your chosen research question (or topic).
The individual entries for your own annotated bibliographies should be no more than one paragraph long. In addition to briefly summarizing the main focus of the work, you may want to include comments on: the author's methodology and scope of research, how the author fits within the larger field of research in that area, and what you think it might contribute to your own argument.
Having a clear picture of what existing scholarship is out there and how it fits together will help you formulate an effective research question or thesis of your own.
OWL (the Purdue Online Writing Lab) offes a number of resources to help with academic writing assignments. Here are their sections on creating annotated bibliographies:
Sample Annotated Bibliographies:
McMahan, David T. "Media Scholarship--From Cartoons to Political Upheaval: An Annotated Bibliography of Recent Publications in Media Studies." Review Of Communication 4, no. 3/4 (July 2004): 295-300. Communication & Mass Media Complete, EBSCOhost (accessed June 12, 2014). LINK
Maya Deren as a Film Theorist: An Annotated Bibliography - a sample annotated bibliography on an experimental filmmaker.