Where is Atticus learn to shoot a gun?

In this younger days, Atticus Finch had the reputation of an expert marksman. He wasn't nicknamed "Ol' One-Shot" for nothing. But Atticus has kept quiet about his special skill for all these years. That's why Scout and Jem are both stunned and impressed when Atticus easily dispatches the mad dog Tim Johnson with a single shot.
Atticus is a naturally modest man, but he had an additional reason for not letting his children know about his shooting abilities. As far as he's concerned his skills are a gift from God, and as such they give him an unfair advantage over other living things. It's notable that Atticus was reluctant to kill Tim Johnson; he only did it because he had to and no one else wanted to take on the responsibility. So despite his very special gift, Atticus won't teach his children how to shoot; he'll leave that responsibility to his brother Jack.