Pick an app on the iPhone that is currently selling well and you feel you could build, then make a competing app. It's boring, but it works and it's a good confidence builder. A lot of the advice in this thread is to build some cool new tech, but I'm gonna say that's a bad idea because it's very risky to do that. Sounds like you're trying to find a reliable way to make money. Also, online offerings will need constant babysitting to make sure the site is up etc. iPhone apps on the other hand can be fully self contained with just little updates for new iOS versions and phones needed.