Starting up is a process consists of series of grueling tasks. The first job of a row of actions is to find an idea that works. But how to find an idea that is not banal, that does not suck and that works is a billion dollar question. Moreover, most of the startup ideas fail; so when you are looking for ideas you have to find one that will work-a toughest job ever. Similarly question comes- how to know which idea is going to work? Paul Graham of Y Combinator has a compelling case to make on both finding start-up idea & finding best one.
The way to get startup ideas is not to try to think of startup ideas. It's to look for problems, preferably problems you have yourself.
The very best startup ideas tend to have three things in common: they're something the founders themselves want, that they themselves can build, and that few others realize are worth doing.
Microsoft, Apple, Yahoo, Google, and Facebook all began this way.