Projects like an annotated bibliography lead you beyond what you knew when you started. Your annotated bibliography will be shaped by what you find and what you've learned, so it make sense to write the Introduction when you know exactly what you've accomplished, and what the final scope and limitations of your resource selection are.
Describe the scope of your bibliography, i.e. whether it covers what you judge to be the best, or the most recent, or a broad sample of the available material on your topic.
Again, does it cover the whole range of opinion, or just one viewpoint or aspect of the topic?