I'm unsure of which version of the text you have, so the page numbers might be off a little bit.
Bob Ewell and his family are brought up very early in the story. Scout is telling her readers about how the Ewell children always show up at school on the first day and then never again. Scout goes home and asks Atticus why she can't have the same arrangement. Atticus then tells Scout a little bit of the family background of the Ewells. He tells Scout on page 31 that the Ewells are granted certain privileges in Maycomb because the rest of the town intentionally turns a blind eye. He then gives the specific example of Bob being allowed to hunt out of season.
He said that the Ewells were members of an exclusive society made up of Ewells. In certain circumstances the common folk judiciously allowed them certain privileges by the simple method of becoming blind to some of the Ewells’ activities. They didn’t have to go to school, for one thing. Another thing, Mr. Bob Ewell, Burris’s father, was permitted to hunt and trap out of season.
Scout is appalled at that and asks why. Atticus then explains, on the same page, that Bob is a drunk, and his family suffers because of it.
“It’s against the law, all right,” said my father, “and it’s certainly bad, but when a man spends his relief checks on green whiskey his children have a way of crying from hunger pains. I don’t know of any landowner around here who begrudges those children any game their father can hit.”
Atticus then finishes the conversation about Bob Ewell by saying that Bob is more or less a lost cause. This quote can also be found on page 31.
". . . he’ll never change his ways."
Page 125 has a great line about Atticus's opinion of Bob Ewell. Scout has just asked Calpurnia why people are upset at Tom Robinson and his family. Calpurnia explains what Bob is accusing Tom of, and Scout says to Cal what Atticus says about the Ewells.
“Mr. Ewell?” My memory stirred. “Does he have anything to do with those Ewells that come every first day of school an‘ then go home? Why, Atticus said they were absolute trash—I never heard Atticus talk about folks the way he talked about the Ewells."
On page 168, Mr. Gilmer is questioning Heck Tate about the night that Bob Ewell reported the supposed crime. The quote is important because it helps to show how racist Bob Ewell is.
Mr. Tate said, “It was the night of November twenty-first. I was just leaving my office to go home when B—Mr. Ewell came in, very excited he was, and said get out to his house quick, some nigger’d raped his girl.”
On page 175, Bob Ewell is being questioned on the stand, and that's when he forcefully and publicly states his accusation against Tom. Again, Bob can't even say Tom's name.
He stood up and pointed his finger at Tom Robinson. “—I seen that black nigger yonder ruttin’ on my Mayella!”
On the next page, readers are told just how proud Bob is of himself for so completely taking control of the courtroom's mood with his statement.
As Judge Taylor banged his gavel, Mr. Ewell was sitting smugly in the witness chair, surveying his handiwork. With one phrase he had turned happy picknickers into a sulky, tense, murmuring crowd, being slowly hypnotized by gavel taps lessening in intensity until the only sound in the courtroom was a dim pink-pinkpink: the judge might have been rapping the bench with a pencil.