To develop applications for iOS, you need to have a Mac, an Apple developer account (to publish to the App Store; costs $100 annually), the free XCode application on your Mac, and ideally some iOS devices to test with.
Experience with either of the programming languages Swift or Objective-C is neccssary, with Swift being the better choice for those without C experience.
println("Hello, world")
is a simple hello world program in Swift.