Get the SDK at the iPhone Dev Center.
Developers get 70% of the revenue from their apps. It costs $99 to publish them on AppStore. Free apps can be listed, at no charge.
Development is done in XCode, only on Mac, and includes a Mac-side iPhone Simulator.
SDK apps will not be able to run until June when the new firmware 2.0 will be out that will also support enterprise applications like Exchange and ActiveSync