The protagonist is a character, usually a main character, who changes or learns a lesson as a result of the events of the story. The protagonist also must resolve the main conflict of the story.
Jean Louise Finch, also known as Scout, is the protagonist of the story. She is also the narrator of the story and so the reader understands all of the characters and events through Scout's point of view. This means if she likes a character, we would mostly see the positive traits in that character. Likewise, if she fears or dislikes a character such as Boo Radley or Mrs. Dubose in the beginning chapters, then we see mostly the negative traits of this character.
When a story is told from the first person (using "I"), then it is a good indication that the narrator is the protagonist. This is because we are experiencing how an important character in the story experienced the events and changed because of them, a key trait of a protagonist. The story is also a story of maturity as Scout grows in age throughout the novel. This is known as a "coming of age" story. It is narrated by Scout as a child (through flashbacks) and occasionally from Scout as an adult. The adult Scout fills in information that Scout learns to help us understand the events. As an adult narrator, Scout gives us information that the young Scout simply could not express to the readers because she is a child.
Scout must resolve several internal conflicts of morality presented in the story. Scout struggles with the conflict of seeing her town's prejudice and the injustice in the Tom Robinson trial because an innocent man is convicted for a crime he did not commit. Her father, Atticus Finch, wants her to learn to make her own decisions instead of relying on the gossip she hears from adults in the town. She learns the lesson that someone should not be judged until you see their actions through their perspective. Scout does this and eventually changes her opinions about several people in the town including Tom Robinson, Walter Cunningham, Mrs. Dubose, and Boo Radley. Most importantly, Scout recognizes that Boo Radley, her neighbor, is NOT an evil man as the town believes. She comes to realize that he is a lonely man who wants friends.
Finally, the events in the story are based on Harper Lee's own childhood and a similar trial to the Tom Robinson trial known as the Scottsboro Trial. I have included a link to a website that explains the Scottsboro trial and includes actual documents relating to the trial.