You don't mention if your monologue has to be written by you or if it can be used verbatim from To Kill a Mockingbird. If you are allowed to use segments from the novel, I would suggest the courtroom summation. It should be just about two minutes or so in length. If you are required to write an original monologue, you might consider several possibilities. You could rewrite Atticus' summation in your own words, using a more modern approach or different circumstances. You might also consider writing a piece where Atticus tells the story of the novel--recounting specific sections that include his children, for example. Another idea might be to picture Atticus 20 years in the future, recounting his family's life since the end of the novel. This sounds like an interesting assignment. Hope these ideas help you to focus on a good one. Good luck.