Developing apps for the Mac or the various iOS devices requires knowledge of Xcode. Xcode is a complete developer toolset that provides all the software necessary to start coding.
The download is free and can be grabbed via the Mac App Store or via developer.apple.com. The interface is quite overwhelming at first if you are unfamiliar with Objective-C and how to create applications using this particular language. Although it all looks complicated on first glance, you can soon become acquainted with the more common features and learn the rest as and when you need them.