Q: How can I control what application automatically starts when I turn on my Xbox 360?

Turn on Xbox 360.

Move to the Settings space (far right option on Xbox Dashboard).

Select System.

Select Console Settings.

Select the Startup and Shutdown option.

Within that option, select Startup.

Pick the default launch application, then click the A (green) button. (See screen shot below.)

Push B until you are back to the main screen of Xbox Dashboard.