A 2005 interview of Mark Zuckerberg contains some great insight into how to become a successful entrepreneur.
Interviewer: Where are you taking Facebook, you can expand to those other schools that you're not at, and then what?
Zuckerberg: There doesn't necessarily have to be more. A lot of people are focused on taking over the world, they're doing the biggest thing, getting the most users, and I think part of making a difference and doing something cool is focusing intensely. There is a level of service we can provide when we were just at Harvard that we can't provide for all of the colleges, and there's a level of service that we can provide when we're a college network that we wouldn't be able to provide if we went to other types of things. I really just want to see everyone focus on college and creating a really cool college directory product that just very relevant for students and has a lot of information that people care about when they're in college.
The moment you try to be the next Zuckerberg, you've already lost. Successful entrepreneurs don't set out to change the world, but start with unambitious projects like Zuckerberg's online directory of university students. Rather than setting out to do something big, start small and focus relentlessly.