To Kill a mockingbird!?!?

K so I am doing a book report on To Kill A Mockingbird. I have already read the book, I just need some like guidance, in other words, I just can't make up my mind, I hsve to choose something for the exposition or beginning of action. When would you consider that to be? I have it narrowed down, but I just need to pinpoint it, when would you say the action starts?