There's a great article over at O'Reilly entitled Build Your First Cocoa App, which walks you through doing just that. If you installed the Dev Tools that come with OS X you can follow this guide and, without writing a single line of code, create a simple Cocoa application. It's a great introduction to the power and versatility that's offered by App Kit and Cocoa. This is the first of a series of articles that will take this simple application and expand upon it, article to article. Even if you don't aspire to be a developer, take the time to check it out.