The passages that you are looking for are going to be found in chapter 12, right in the middle of the chapter. Calpurnia ends up taking Scout and Jem to her church, because Atticus is out of town. She doesn't trust them to go to their own church unsupervised; the last time that they did that, Jem caused a ruckus by convincing a little girl that if she touched the radiators "she wouldn't get burnt if she had enough faith." So, to Cal's church they went. They meet Zeebo, who happens to be "the garbage collector," so, a garbage man. But, once church starts, they realize that he is also in charge of leading the congretation in hymns. And by lead, it means that he sings out each line, pauses, and waits for the rest of the congregation to repeat each line. He does this for two reasons: 1. Most black people couldn't read, so, they just repeat what he says because they can't read a hymnal, and 2. The church might not have enough money to provide hymnals for all of its members. So, Zeebo is not only a trash collector, he is a literate hymnal leader and singer too.
I hope that those thoughts help; good luck!
It is also important to know that Zeebo is Cal's son (but don't think of a little boy... he's a man). Zeebo is also the "collector" of the body of the rabid dog that Atticus shoots in the street.