Writing the code
Putting together a project using XCode is not difficult. However, having someone show you the process in real time will help you understand what to expect when writing your code.
The following video is the second in a series demonstrating how to use XCode to add your Cocoa and Objective-C code. The big problem is the graininess of this presentation. If you listen closely, though, you should be able to follow along without much difficulty.
Creating a game
Moving on now. The real key to becomming a great app developer is coming up with a good idea. 1% inspiration and all that.
The following video series is very in-depth, but will show you tips and tricks for navigating XCode. And, hey...game!