If you want to learn how to code in Apple's version of C (which is Objective-C), then learning Objective-C wouldn't be a bad idea. Learning Objective-C will allow you to develop pretty much everything Mac-related (iPhone, iPad, iPod, and Mac applications). In addition, it will show you how to write C code "the Apple way". It may be a little confusing at first and take some time to learn, but you'll get the hang of writing C code "the Apple way".
Once you know some Objective-C, the Cocoa Touch framework will open a whole new door of you to iOS app development. Cocoa touch is the framework you use to develop for iOS devices (It has the all the functions you want and need for your app to get the most out of your iOS device). If you have some programming experience already, take a look at Objective-C just to get a feel of the language then you can jump into learning the Cocoa touch framework. If you're new to programming, spend some time studying Objective-C so you know/understand everything about the language then you'll be set for using the Cocoa Touch framework.
