iPhone app development is a broad topic, so it's often hard for developers to know where to begin. The best way to get started is to familiarize yourself with some of the essential features of the Foundation framework. These include value classes such as NSString, NSNumber, and NSDate, collection classes such as NSArray, NSDictionary, and NSSet, and utilities like NSBundle, NSEnumerator, and NSAutoreleasePool, plus such core concepts and design patterns as property list programming, mutable subclasses, key-value coding, and key-value observing.
What you will learn
You should also get a solid grounding in memory management techniques and Objective-C 2.0 declared properties. Learning Objective-C can be a major hurdle in getting up to speed on iPhone development, even if you're familiar with other mainstream object-oriented programming languages. Objective-C's powerful dynamic runtime system makes the language behave quite differently from most of its peers, with some surprising (though very cool) consequences.