One of the best techniques for humor in this novel is the fact that Scout, as the storyteller, so often repeats the "legal jargon" that she has grown used to as a result of living with a lawyer.
The best places to find examples of the legal diction, I find, are in sections of description, as Scout often uses legal words in retelling stories from the past. Also, you might review the trial scene (Chapters 16-20) as it will undoubtedly have a few words and phrases that are specific only to courtrooms.
In the meantime, here are a few to help you get started:
From Chapter 1 (The story of Boo Radley's time of juvenile delinquency): "disorderly conduct," "disturbing the peace," "assault and battery," "using abusive and profane language in the presence and hearing of a female." All of these are actual legal charges though the last one, clearly, is outdated.
From Chapter 16 (Description of Judge Taylor): "champertous connivance." This is a legal term that basically means that one party assists another party in a lawsuit and shares in the proceeds.
From Chapter 17: "defendant," "cross-examination," "testify."