my head in town, I couldn’t represent this county in the legislature, I couldn’t even tell you orJem not to do something again.”

This sentiment -- and it is repeated in Chapter 11 when Atticus states, "This case, Tom Robinson’s case, is something that goes to the essence of a man’s conscience—Scout, I couldn’t go to church and worship God if I didn’t try to help that man" -- reveals the depth of Atticus' conscience and commitment to do what he believes is the right thing irrespective of its popularity and the ridicule to which it will inevitably expose his family. It is later in the series of exchanges Atticus has with his family, including Atticus' brother Jack, in Chapter 9, that Atticus refers to his appointment to this case by Judge John Taylor:

“Before I’m through, I intend to jar the jury a bit—I think we’ll have a reasonable chance on appeal, though. I really can’t tell at this stage, Jack. You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, ‘You’re It.’”

It is in Chapter 9, therefore, that Atticus reveals that he has taken this unpopular case because he was appointed to it by the presiding judge. Atticus could have, conceivably, turned down the judge's request that he defend Tom Robinson, crippled, desperately poor African American. It is emphasized, however, that he accepted the case because of a moral imperative to display for his children and to others his commitment to do what he believes is right.

To Kill A Mockingbird by Harper Lee reveals how justice, for the people of Maycomb County, is not about fairness but about serving what this community sees as its specific needs, regardless of the rights of Tom Robinson. Harper Lee exposes the justice system and its dependence on the community in delivering a fair verdict; something that is impossible in Maycomb County because of deep-set discrimination, stereotypes and mistrust. Racial prejudice dominates the actions of the community and Tom Robinson's guilt is a foregone conclusion in the minds of the community despite the fact that they know that the Ewell family and especially Mayella's father, has questionable integrity.

Atticus knows from the beginning that he has no likelihood of successfully defending Tom. Tom's innocence is almost immaterial but as Judge Taylor appoints Atticus to defend Tom, there can be no discussion. Atticus mentions that, "John Taylor pointed at me and said, 'You're it.'" It is between chapter 8 and chapter 9 when this apparently takes place and in chapter 9 the reader becomes aware of Atticus's determination to do whatever he can. However, even the fact that Tom admits to feeling "pity" for Mayella Ewell will ultimately prove to be too much for the all-white jury to contend with and Tom will be found guilty. 

Unlike the film version of To Kill a Mockingbird, which depicts Judge Taylor coming to Atticus's house to personally ask him to defend Tom Robinson, the Harper Lee novel has no specific meeting. The first mention of Atticus's acceptance comes during a conversation with his brother, Jack. He informs his brother that an acquittal is an impossibility, but

    "Before I'm through, I intend to jar the jury a bit--I think we'll have a reasonable chance on appeal, though... I'd hoped to get through life without a case of this kind, but John Taylor pointed at me and said, 'You're it.' " 

(This is found in near the end of Chapter 9.)

Oddly, this conversation between the two brothers is not overheard by Scout, so it is not part of her normal narrative. She does not learn that Atticus has been handed the case (rather than volunteering for it) until just before the trial.

    "Lemme tell you somethin' now, Billy," a third said, "you know the court appointed him to defend this nigger."
    ... This was news, news that put a different light on things... 

(This is found midway though Chapter 16.)

We are not told specifically when Atticus agrees to take the case, but as the previous answer makes clear, we first learn of it in Chapter 9. This is the first intimation of the difficult times that lie ahead for the Finch family over the case. Atticus counsels Scout to exercise restraint, because the case is going to cause a lot of negative talk and he knows that she is fiery and liable to lash out at the first provocation. He knows there will be plenty of provocation in the days ahead, and he wants her to meet it with dignity.

You might hear some ugly talk about it at school, but do one thing for me if you will: you just hold your head high and keep those fists down. No matter what anybody says to you, don’t you let ‘em get your goat.

This is perhaps the first major life lesson that Scout has to learn. Of course, she doesn't learn it all at once, and it's not long before she's fighting her aggravating cousin Francis over the same subject. 

The introduction of Tom Robinson's case in this manner - through Atticus's explanation to the young Scout - is quite appropriate, as of course the whole novel is seen from the young Scout's perspective, with some seasoned remarks from the older Scout in retrospect. The circumstances of the case are not immediately made clear, as Atticus tries to distill it into terms that a child could understand. At this stage, all we really know is that it's something that's going to spell trouble for the Finch family.

There is no specific scene in the book that depicts Judge Taylor directly asking Atticus to defend Tom Robinson. However, in chapter 9, Atticus describes how Judge Taylor asked him to take the case. Atticus is discussing the upcoming trial with his brother and says,

I really can’t tell at this stage, Jack. You know, I’d hoped to get through life without a case of this kind, but John Taylor pointed at me and said, "You’re It" (Lee, 91).

Leading up to the trial, both Jem and Scout are under the impression that their father made the independent decision to defend Tom Robinson on his own. In chapter 16, the children enter the crowded Maycomb courthouse and attempt to look for a good seat to watch the trial. Unfortunately, Scout gets stuck by the back staircase, where she overhears a group of old men and courthouse critics, known as the Idlers’ Club, discussing the trial. Scout is astonished when she overhears one of the men say,

Lemme tell you somethin' now, Billy . . . you know the court appointed him to defend this nigger (Lee, 165).

After Scout overhears the man's comment about Judge Taylor making Atticus take the case, she finds it strange that her father never said anything about it.

The Tom Robinson trial begins in Chapter 17 and ends in Chapter 21.

Chapter 17: Sheriff Tate is the first person called to the witness stand. Atticus thoroughly questions him regarding the location of Mayella's injuries. Tate says that Mayella's right side of her face was badly beaten and was predominately bruised. The next witness to come to the stand is Bob Ewell. The prosecutor, Mr. Gilmer, asks Bob to explain what happened on the night of November 21st in his own words. Bob claims that he heard his daughter scream and says, "I seen that black nigger yonder ruttin' on my Mayella!" (Lee 231) Ewell tells Judge Taylor he saw Tom having intercourse with his daughter. During Atticus' cross-examination, Bob confirms Sheriff Tate's testimony that Mayella's right eye was bruised. Atticus then asks Bob to write his signature on a piece of paper. Atticus does this to show the jury that Bob is left-handed, which suggests that he could have possibly beaten Mayella.

Chapter 18: Mayella Ewell takes the witness stand. She testifies that Tom Robinson assaulted and raped her. The audience learns about Mayella's lonely, pitiful life taking care of her siblings and living with an abusive father. Mayella seems confused and cannot recount whether Tom beat her or not. Her testimony seems fabricated, and she refuses to answer any of Atticus' questions regarding where the children were during the assault and who she called for while she was being assaulted.

Chapter 19: Tom Robinson takes the witness stand. He tells Atticus that he was familiar with Mayella because he helped her plenty of times with her chores. Tom says that one day she asked him to fix an old door that kept falling off the hinges. He claims that while he was inside the house, Mayella grabbed him around his waist and kissed him. Tom says that he became scared and ran out of the house. Mr. Gilmer cross-examines Tom and Tom admits that he helped Mayella because he felt sympathy for her. This comment does not sit well with the jury and Gilmer proceeds to talk down to Tom.

Chapter 20: Atticus gives his closing remarks and begs the jury to judge Tom based off of the testimonies and evidence presented. He urges them to look past the fact that Tom is black and encourages them not to be swayed by their prejudiced beliefs.

Chapter 21: At the end of the chapter, Judge Taylor reads the verdict. Tom Robinson is found guilty of raping and assaulting Mayella Ewell. Atticus walks quickly down the middle aisle to exit the courtroom once the verdict is read.