Justice is not a simple concept in this novel. Though the children, Jem and Scout, are saved from an attack in the end by a man who has been unfairly judged by the town, the book also features a trial in which a man is wrongfully convicted of a crime. Justice, often seen as a fair result or reaction to actions taken, is here presented as an ideal that is culturally compromised.
I would agree that (sometimes) the best way to define or examine something is to look at its opposite. Many of the characters are treated unjustly in the novel. One of the main themes in the novel is prejudice--prejudices are defined by the injustices done to people. Another suggestion would be to compare and contrast the characters who are treated both with justice and without.
You might want to discuss some of the many different ways in which people are treated unjustly in this novel. Tom Robinson is the obvious example of someone who fails to receive just treatment, but numerous other characters are not treated justly throughout the story. You might want to consider what justice means in this tale. Atticus's famous comments about walking in other people's shoes suggests that something like the Golden Rule is implied: do unto others as you would have them do unto you. According to this standard, there are many large and small examples of injustice in this novel.
Justice is portrayed as being complex, and based on society. The legal system does not get justice for either Mayella Ewell or Tom Robinson, but Boo Radley kills Bob Ewell and therefore gets justice for them both, and for the children.