In order to create your own iPhone applications, you need to have certain tools and certain programming skills. Apple has made iPhone application development very easy and made the tools available to everyone at a small fee. However, the iPhone SDK is available only for Macs, not for PCs.
The iPhone SDK will work on any Intel based Mac running OS X Leopard (10.5.6 or later). It's preferable to have a powerful Mac with lots of RAM.
Once, you have a Mac, you just need to Download the Apple iPhone SDK from http://developer.apple.com/iphone/ and install it on your Mac.
Just create an Apple ID for free and download the iPhone SDK.
To publish your application to the Appstore, you need to pay $99 and register to the Standard iPhone Developer program.
It will allow you to develop, test and distribute your application on the Appstore. It also gives you access to all the documentation, tutorials, videos and sample code that you will need.
It's also preferable to have an actual device to test your application - an iPhone or an iPod Touch. Though the SDK includes a simulator to run your apps, it's always better to try it out on the real thing before publishing it.