First we need to open Xcode, so once you have the SDK installed, you'll need to open /Developer/Applications/ and look for Xcode.app. This is Apple's IDE (Integrated Development Environment) that allows you to code, debug, test, and build all of your iPhone and Mac applications. When you open this application, nothing specially really happens, although you might see the welcome center -- if you see this, you can choose to disable it at startup by using the check box at the bottom.
To create a new project, select File > New Project. In the resulting window select iPhone OS Application > View-based Application, and click "Choose." You will then need to specify a save name and location for the resulting files that will combine to create your application. In the resulting Xcode window, you should note that most of the work is already done for you!
At this point you have a fully functional application. Try it out: click the "build and go" button at the top of the window and wait while the app is compiled and opens in the iPhone Simulator. The app definitely doesn't do much, but still, it's a running application you made without writing any code.
Continue reading to learn more about Xcode, and get a brief UI overview.