I did this by first running through Apple's Currency Converter tutorial (on Apple's dev site). I made sure that I understood completely what every part of the tutorial was doing, how it worked, and why. I did this by referring regularly to Apple's objective-C specification, the document that covers how the language itself works, and why. I also referred to the Cocoa API docs regularly, which gave me an idea of how Cocoa is structured, and generally, how cool it is.

There were a couple of nice "eureka" moments. Figuring out that objective-C messaging is not equivalent to c++ method calls or C function calls was one. Figuring out that runtime introspection was something I both could and should rely on was another.