How to develop iphone application?

To develop iOS apps, you need:

• A Mac computer running OS X 10.8 (Mountain Lion) or later
• Xcode
• iOS SDK

Xcode is Apple’s integrated development environment (IDE). Xcode includes a source editor, a graphical user interface editor, and many other features. The iOS SDK extends the Xcode toolset to include the tools, compilers, and frameworks you need specifically for iOS development.

Which programming language should I know?
In order to develop IOS application, you need to know objective -C. It is similar to C++ programming language or C programming language with object oriented features.