Storyboard apps use the key UIMainStoryboardFile, or “Main storyboard file base name”, to specify the name of the storyboard that must be loaded when the app starts. When this setting is present, UIApplication will load the named storyboard file and automatically instantiates the first view controller from that storyboard, then puts its view into a new UIWindow object. No programming necessary.
You can also see this in the Project Settings screen in the Deployment Info section