Obligatory pg essay: http://www.paulgraham.com/startupideas.html. Start here.
Live in the future, then build what's missing.
I also draw inspiration from Bret Victor's[1]:
If there's something in the world that you feel is a wrong, and you have a vision for what a better world could be, you can find your guiding principle, and you can fight for a cause. So after this talk, I'd like you to take a little time, and think about what matters to you, what you believe in, and what you might fight for.
[1] http://gumption.typepad.com/blog/2012/03/principle-centered-...