You need to be a member of the iOS Developer Program.
This is a $99 expense per year that is absolutely mandatory if you have any hope of creating an app, submitting it to Apple, and raking in any revenue.
Joining gets you access to the iOS SDK, lots of resources in Apple's iOS Dev Center, and the ability to run your in-development app on an iOS device.
You can learn more here.
You need to know how to program.
iOS apps are written in a programming language called Objective-C and they use a set of tools called the Cocoa Touch API to interpret a user's touch input.
If you're already familiar with these, then you're well on your way.
If not, well, see the next slide.
If you don't know how to program...
...there are countless resources out there to get you started with programming in Objective-C. Try a book from Amazon or your library. Find a tech-savvy friend willing to spend some time with you and get you up and running. And there are awesome sites like Stack Overflow to get you unstuck when you need help.
Can't program but still want to make an app yourself?
If you can't program but still want awesome results...
...you'll be better off hiring an experienced freelance developer from a site like ELance. When in doubt, go with a seasoned pro who will help see your project through.
If you're building the app yourself, you need a programming environment.
An Mac environment called Xcode is probably your best starting point.
It's fully-featured enough to handle most everything you'd need to put together a straightforward app. You won't have access to some of the more technical resources, such as low-level libraries and drivers.
The best part? It's free in the Mac App Store.
Submit your app for Apple's approval.
Apple is less than transparent with its app approval process, but every app in iTunes submits to it.
Apple is basically looking to keep out malicious content and ensure that every app meets its usability guidelines – are the buttons and other aspects of the interface big enough and easy to use?
If you've done your due diligence, you'll be fine.
Once it's all done, PUBLICIZE!
No one can buy an app they don't know exists. Get your name out there on Twitter and Facebook.
If you want to take an aggressive, professional approach, look into hiring a PR firm.