Once your purchase the license to develop for Apple ($99) you are ready to download Xcode, which is the program used to develop you Apple apps. Xcode is a development tool created by Apple. With Xcode you can create your app, test it with an emulator, and publish it to the iTunes app store (after extensive apple review of course).
How difficult is Xcode? If you are new to programming, its going to be very difficult. Xcode allows you to use C/C++ to program your app. In addition to knowing the language you need to understand Xcode interface as well as design for iPhone/iPhone, so it can get complicated very quickly. For the experience object oriented programmer, Xcode should not be too difficult to pick up. Overall, this software is not made for designers, rather experienced programmers.