iOS programming requires knowledge of Objective-C or Swift, and the Xcode IDE. Apple provides many resources to help developers learn their platforms, including Xcode tutorials, the Apple Developer website, and the Swift Playgrounds app. Other popular resources include online courses and books.
As with any programming language, the best way to learn is by writing code. Experiment with different features and functions to get a feel for how they work. Try to build small apps or components of larger apps to get a better understanding of the development process. When you run into problems, search online or ask experienced developers for help. With time and practice, you will become an expert iOS programmer.