If this is an exam question, I will try to help you along in strategizing an answer. Amir is the protagonist of the novel, so the entire story is primarily about him. The best thing you can do is to start with what you thought of him as a person as you read. Once you've established that, you want to go back into the book and figure out what made you feel that way. Here's an example of what I mean:
I feel that Amir is portrayed as weak and pathetic. Amir makes a decision early in the book that is the basis for the story. He decides not to help his friend Hassan when he is being gang-raped by a group of boys in an alley. Given that he is very young and scared, this is somewhat understandable. I can't say that I would have done any differently. However, Amir does not take the opportunity to fix his decision, to apologize or to help Hassan through what had to be an horrific experience. When Amir is asked by Hassan's father if he knows what is wrong with him, he replies: "How should I know what's wrong with him?... Maybe he's sick... Now am I going to freeze to death or are you planning on lighting the stove today?" Not only is he lying, he's condescending to Hassan's father.
As you can see, I stated my opinion and I backed it up with support from the text.