After writing a motivator, something which will hook your reader, such as the quotation from the novel that cites Atticus Finch's caution in Chapter 10 to the children against killing a mockingbird because it is a sin to harm a creature so innocent, you can lead into the thesis at the end of your introductory paragraph. The composition of this thesis can revolve around the idea that Harper Lee's novel contains metaphoric mockingbirds in the persons of Boo Radley, Tom Robinson, and a third character such as Helen Robinson, or even Mayella Ewell or Mrs. Dubose, who both utter cruel judgments out of extenuating circumstances. Thus, the three subtopics will be the mention of the three metaphoric mockingbirds whom you select and why they are these mockingbirds.
In considering your choice for these metaphoric mockingbirds, you may wish to peruse chapters that contain characterizations of the persons. That is, look for the actions and words that these characters use as well as the motivation behind these words and actions. For instance, if you choose Mayella as a mockingbird, you may wish to consider that until the trial, she intended no harm to Tom; instead, she simply craved human warmth and affection. Clearly, then, her accusations of Tom Robinson are due to her having been threatened by her unconscionable father who intimidates her into vilifying Tom and intimating that no Negro has integrity. (For support of this look at her behavior and words at the trial.)
So, in the subtopic you will present the name of the metaphoric mockingbird with a general statement of why this character is an innocent, a harmless person. Then, with reason, citations, and examples of incidents from the narrative of Lee's novel, you will support your contention that this particular character is, indeed, a mockingbird. In essence, yours is an argumentative essay, for you must prove that 3 characters represent the metaphor mockingbird of Harper Lee's novel.