Hey, you need to think of a life lesson from this novel first. There are many. Here is one that I appreciate:
You never really understand a person until you consider things from his point of view . . . until you climb into his skin and walk around in it. - Atticus
So, for your opening paragraph you might write:
"In Harper Lee's novel To Kill a Mockingbird, there are many life lessons to think about. One of the lessons is also a larger theme in the novel. It is sated to Scout by her very wise father Atticus, "You never really understand a person until you consider things from his point of view . . . until you climb into his skin and walk around in it. Much of the novel examines this theme. As Scout matures, she begins to understand this in this in the context of her life.
Then pick three main points. You could consider Boo Radley, Mayella Ewell and Mrs. Dubose. Then, in a seperate paragraph, explain the idea related to each character. Find a quote for each main idea in the book to back up your statements. Then write a conclusion. It is difficult to explain essay fomat on this forum but I hope this helps a little.