There is actually some seemingly contradictory evidence concerning how Scout first learned that Atticus is appointed to defend Tom Robinson at his trial in To Kill a Mockingbird. In Chapter 9, Scout is deliberately eavesdropping on Atticus' conversation with Uncle Jack. Atticus tells Jack that
"... John Taylor pointed at me and said ' You're It.' "
Apparently Scout does not understand this reference that Judge Taylor has assigned Atticus to the case, rather than Atticus seeking it out on his own. In Chapter 16, however, Jem, Scout and Dill all sneak down to the courthouse where they overhear several men discussing the trial. When they hear Atticus' name mentioned, they pay closer attention. Scout learns that
"... you know the court appointed him to defend this nigger."
"Yeah, but Atticus aims to defend him. That's what I don't like about it.
This was news, news that put a different light on things...
This is Scout's first inkling that her father was appointed to the case, despite what she had overheard at Finch's Landing.