Good choice for now.
In your Xcode app, there should be a window in the center-bottom area called 'All Output'. This might have error messages that might tell you why the app exits.
Are you testing just the basic template?
When the simulator is open, you can delete the app just like it was a real iOS device: click your mouse on it and hold it down until the icons start jiggling and a red X shows up in the upper left corner of the icons; click on the red X for the app and delete it (a pop-up will ask you to verify that).
After deleting the app, try running it from the Xcode interface again. Then, close it from the Xcode interface (Apple splat key + period). Then try to run it from the simulator directly, it will have been left installed there.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master