In Harper Lee's novel, To Kill a Mockingbird, the narrator, young Scout Finch, is like a sponge when it comes to picking up information and advice from others. Below are examples of influential information that she gains from her friends in Maycomb.
JEM FINCH. Jem teaches Scout that Atticus' trust is worth keeping; she also discovers that "school's different" than life at home.
ATTICUS FINCH. Above all, she learns from Atticus to step into the other person's shoes to see things from their perspective.
MISS MAUDIE ATKINSON. She shows Scout that true friendship does not have to include prying into each other's personal business. She also tells Scout that her father has hidden talents, and that people who have them have no reason to brag about them.
CALPURNIA. In addition to learning how to write in cursive, Calpurnia teaches Scout that flaunting a person's education and intelligence is not always necessary, especially around others lacking in those skills.
MR. WALTER CUNNINGHAM. Scout understands that though a man may have little money, he can be honest and still pay his debts the best way he can.
The above answer gives some good points, but since To Kill a Mockingbird is a big book, there is much more to say.
Jem teaches Scout that it is ok to grow up. One of the characters that changes the most is Jem. He becomes a young man at the end of the novel. By his transformation, Scout is also able to grow up. Hence, we can say that Jem's influence is indirect. His change leads to her change.
Atticus teaches her more than we can imagine. So, let me limit myself to one central point. He teaches her courage. In one conversation Scout asks if Atticus is going to win the trial. He says "no." Scout is perplexed. Atticus, then, explains that it is important to stand for what is right even if you will lose. Atticus also reinforces his teaching by defending Tom Robinson to the best of his ability.
Miss Maudie teaches Scout many things as well. However, she teaches the children that it is a sin to kill a mockingbird. Even though Atticus says this first, Miss Maudie drives it home, and in some ways Scout knows this lesson better than Atticus at the end of the story.
Mr. Cunningham teaches Scout that even good men can do "evil" things. I'm referring to mob scene where Mr. Cunningham seems to be leader who wants to hurt Tom Robinson. Moreover, she learn that people can change through him, as he goes away when he comes to his senses.