The book opens in the adulthood of Amir and he relates then and there that he made a bad decision in his childhood and he has spent the rest of life to that point trying to redeem himself. Then the book slides into a flashback and we learn what bad decisions he makes, namely his poor treatment of his friend Hassan, and his not trying to stop Assef's attack of Hassan after the kite running contest. He reveals his guilt immediately after the attack when he talks about why he didin't react; he reveals it in his subsequent horrible treatment of Hassan and his false accusations; he reveals it in a comment here or there throughout the novel as he tells about leaving Afganistan and his life in the United States. He specically thinks about it when he learns about the mistake his wife made in running off the man. She can reveal her sins, while Amir states that he just can't.
His guilt hits him hardest when he hears from Kahn and learns that he needs to return to Afganistan. Once there when he learns that Kahn knew the whole truth all this time and that Hassan is actually his half brother, the guilt is almost overwhelming and he is driven to atone for that past by doing everything in his power to save Sohrab from the Taliban men and Assef. He devotes himself to bringing Sohrab home and healing Sohrab's spirit in a way he never did for Hassan.
There are references to his guilt in almost every episode of the novel -- it is so clearly a novel about guilt and redemption, and it works well because we are never left too long without a reminder of what Amir did and his feelings about those childhood actions.