There is no specific point in the novel at which someone approaches Atticus asking him to defend Tom Robinson. In Chapter 9 Scout accosts her father one evening asking him, "Do you defend n---s?" because Cecil Jacobs has hurled some insults toward her about her father. Later, in Chapter 9 when Atticus and his family visit his brother, Jack asks Atticus about the case: "Atticus, how bad is this going to be? You haven't had too much chance to discuss it[the case of Tom Robinson]."
It is interesting that the subject of Atticus Finch's defense of Tom Robinson comes subsequent to a chapter whose subject is Boo Radley and the children's stealing into the yard of the Radley's and their discovery that Mr. Radley does not want their interference as he has cemented the hole in the tree where they have put things. Thus, the reader unconsciously connects these two men and the mockingbird motif begins.
There is no specific answer to this question.Atticus basically defends tom robinson and his case because he's an lawyer. And atticus is not races calling tom a nigger like everyone around him that is critisizing tom.