If your TV supports HDMI-CEC one touch play, it can be enabled in SHIELD TV Display & Sound > Power control > CEC TV On settings to turn on your TV and switch to the input that SHIELD TV is on whenever your SHIELD wakes up or you press a button on your SHIELD Controller or Remote.
Your TV can also be configured to sleep when your SHIELD is turned off by enabling the Display & Sound > Power control > CEC TV Off switch.
NOTE Most TVs have HDMI-CEC "turn TV off" disabled in the TV settings menu. HDMI-CEC (One Touch Play and TV off) must be enabled in your TV's settings. Search for your TV manufacturer's name and HDMI-CEC to find out what they call it and how to enable it.