The introduction of any academic essay needs to get several jobs done.

Firstly, it needs to pique the reader's interest and make them interested in what you have to say. Secondly, it needs to introduce the argument that you are going to make or the approach that you are going to take, always reflecting back on the question that you have been asked. Thirdly, it should provide any relevant background information without really getting into the "meat" of your discussion or argument.

Your opening sentence is the ultimate hook that draws the reader into your essay. For the topic you have been given, perhaps consider something along the lines of the following:

Kite flying was a common pastime for Amir, a young boy growing in Kabul. He could never have predicted the impact that this activity would have on his life or that kite running would become so symbolic of friendship, a society in peril, and the tenacity of the human spirit.

In writing specifically about this novel and your comment in the original post about having to discuss the title of the novel, your introduction's first sentence should  talk about kites!  You can talk about a childhood memory of kites; you could talk about what you learned about the kite-running contest from the novel, you could talk about how the kite is symbol of Amir and Hassan's relationship through the novel.  Like stated above, your choice will depend on what the rest of the essay will do.  This is a great topic for an essay!

Your opening sentence will depend on the angle you take with the essay.  Once you have a purpose (inform, persuade, entertain, etc.), you will know more about the tone you want to take and the "so what?" of your paper.  Brainstorm about what you want to say and how you want to say it.  Then focus on the ways you might begin your paper which will prove to be an effective lead and hook.

This is of course a very open question because it depends a lot on what particular slant you are going to approach your essay question with. Of course, the most important thing to do is to think of an introductory sentence that will grab the attention of the person reading your essay and entice them on into reading the rest of the essay. You might want to think about commenting on how the title in some ways seems inappropriate to the rest of the action of the story - after all, flying a kite seems a very peaceful, enjoyable image, which stands in sharp contrast to the actual contents of the plot - child rape, betrayal and physical abuse all played out against the backdrop of the Taliban rule of Afghanistan. I would be tempted to start my essay as follows:

"Nobody would ever believe that the innocent diversion of kite flying could lead into an epic tale of betrayal and eventual redemption, however Hosseini, in his novel The Kite Runner, manages to weld this activity with the journey of one man from betrayer to his redemption."

That is just one idea of course - you might find this good to think about this as a starter to stimulate your own ideas.