The first five paragraphs of Harper Lee's To Kill a Mockingbird tell the readers everything they need to know about the setting and tone of this novel, at least in the beginning.
First, nearly every one of the major characters is named: Jem, Dill, the Ewells, Atticus, and Boo Radley. Though we do not know the narrator's name, we know Jem is her brother and their father is Atticus. We gather that Atticus is a fair man (note how he settles a dispute between his children), Jem is a typical teenage boy, and Dill is a real character. The Finches have quite a family history.
Second, we have a sense already about the narrator and time. This story is being told in hindsight (looking back) from the perspective of one of the characters in the story. The time of the novel is suggested in the following lines:
the Landing nevertheless produced everything required to sustain life except ice, wheat flour, and articles of clothing, supplied by riverboats from Mobile.
The story is set in the days of riverboats, though that does encompass many years.
Third, we get the rather facetious account of the Finch family history, something which will be quite important to at least one character (Aunt Alexandra) later in the story. We know the story is set in the South, probably in Alabama, since Mobile is mentioned.
There are a few things we do not know that we find out later, such as the specific town and year, and there are some key characters we do not yet meet; however, the details and tone of the opening paragraphs are a clear indication of what kind of story the readers can expect.