This is a simple question from interviewer. Some people make the mistake of answering it wrong by saying that I was looking online for job and found your company. It does not give a good impression. You have to make the interviewer feel important by telling how you found this opportunity and how it fits your career goals.
If you have a friend in the company, then you can mention that as well now.
Sample answer is:
I have always wanted to work in Mobile commerce domain. I researched on the internet and found your company to be pioneer in this domain. I have studied about the products and services that your company has launched in the past. Your company and this position looks a great fit for my career goals.