First, you need an Apple computer. Then you need the Xcode development environment, also from Apple, which is free and runs on your Apple computer. Then you need to know Objective C, which is the programming language, and know about event-driven programming. Then you need to understand Apple's recommended design patterns for developing solid apps for the iPhone OS, including the model-view-controller pattern.
If > half of the above sounds like it's from another planet, then you'll have a steep learning curving ahead of you. If you're a professional software developer, then you'll know most of the above already.
Apple's developer site has a pretty good tutorial, and of course there are many books.
(I would give it a go, except I don't have an Apple computer!)
"Call the police in the United States of America!"