An iphone app gets its start from a single thought or idea. Not just a half-assed or half-baked idea, but a solid, good idea that you can actually picture to go somewhere. Think slightly long-term here, that's all right. Will it be useful? Will it solve a specific problem? Can it solve problems that are not so easily solved through other means? If it's not for problem-solving, will it entertain or amuse people?
While others come up with original ideas for apps, there are others who prefer to take their cues from existing apps and create something better. If that is the case, are you sure that the enhancements are so significant and can actually be referred to as legit upgrades of the existing app?
Then you should sit down and make an assessment of your resources. Do you have, at your disposal, the needed tools for creating an iphone app? It will be easier if you list down the tools that you need and the tools that you have. You can refer to sites dedicated to teaching how to develop an iphone app to know the necessary tools you would need. You will then have a clearer idea of those that you don't have and get started looking for them.