Try changing the video resolution of the laptop to the native resolution of the TV.
You may also have to change the order your boot the laptop and select the input on the TV. They have to "handshake" and that can get tricky. It could take 5-10 seconds.
After you get the mirrored desktop up on the TV you want to then set the TV as a second screen.
Set the laptop display back to what it was before. Your graphics properties control panel will allow you to do this as well as make other adjustment to get the best picture on the TV without messing up the laptop display.