There are several thought-provoking themes developed by Lee in this novel; trying to decide which one to explore in and of itself could be a challenge. The most obvious one, perhaps, is the theme of racial oppression and prejudice as it still existed in the Deep South in the 1930's. The trial of Tom Robinson, which, for all intents and purposes should have been decided in his favor, but was not simply because he was a black man provides an excellent starting point for an exploration of the social structure of the Deep South, a structure where a man as repulsive and despicable as Bob Ewell could consider himself "above" someone as well-meaning and honorable as Tom Robinson appeared to be--simply because Ewell was white.
Another theme to consider, although less obvious, might be the deep faith Atticus had in human nature, and whether or not it was wise, realistic or justified. Although Atticus is a man of deep integrity and believes strongly in putting himself in another's place before passing judgment, his habit of seeing the best in people and overlooking the worst almost got his children killed at the hands of a drunk Bob Ewell.
A third idea might be to explore the nature of Southern society as depicted by Harper Lee, who is credited by some with capturing the flavor and essence of the Southern way of life as well as William Faulkner. Looking closely at the words and actions of Aunt Alexandra would be a good way to start this essay, particularly when she hosts the missionary society meetings, where one gets a real sense of the "everyone knows everyone's business" lifestyle led by residents of small Southern towns.