Who were his first two clients? What happened to them? Why? How did this case change Atticus as a lawyer?

From the text:

His first two clients were the last two persons hanged in the Maycomb County jail.

They persisted in pleading Not Guilty to first-degree murder, so there was nothing much Atticus could do for his clients except be present at their departure, an occasion that was probably the beginning of my father’s profound distaste for the practice of criminal law.