What is Harper Lee trying to achieve in part 1 of To Kill a Mockingbird?
Lee is establishing the unconventional relationships between the Finch family. Atticus Finch, father of Jem and Scout, is a sympathetic & sensitive lawyer who has raised his children to see, in the words of Martin Luther King, Jr. "not the color of one's skin, but the content of one's character."
Growing up in Maycomb, Georgia in the 1930s, this was not an easy thing for Atticus to teach. The South was still very segragated. Atticus' political leanings were suspect among the "old" Southerners, as was his method of raising children. When Scout goes to school, she is ridiculed for his method of teaching her.
As the novel continues, the morals Atticus has taught Scout & Jem about not making judgments about people are challenged by the mysterious "Boo Radley." Boo is never seen by the children & thus a source of speculation. He is kind, however, leaving the children little gifts like chewing gum in a holllowed out section of a tree on his property. His generosity begins to make Scout think about "content of character."
When Atticus decides to take on the case of Tom Robinson, a black man accused of raping a white woman, the Finch children will come under more scrutiny for their liberal beliefs. Scout & Jem have been provided a firm foundation by their father, but it will be up to them to weigh what they have learned about people & the world in which they live.