Press TV button on controller, power on television, select input / applicable volume / etc., power on console.
Just three! At the risk of sounding like late-night TV pitchmen, we must emphasize how important this is. It remains impressive after days of using the console at home. Ever since we (as a society) first plugged computers called "gaming consoles" into our televisions, we've been following an archaic process of swapping remotes and pushing buttons and all sorts of needless busywork. Nintendo's Wii U tablet controller thankfully streamlines that process in a small, but tremendously meaningful way: during initial console setup, you're prompted to sync your television and the tablet controller, allowing the tablet itself to control the TV's power, volume, channels, and input directly.
And let's pause to emphasize the importance of this step as part of the initial console setup, rather than background functionality to be sussed out later on -- this fundamentally transforms how the vast majority of soon-to-be Wii U owners will interact with their Wii U. The console instantly becomes the de facto media unit, interacting with your cable box, DVR, and various digital streaming offerings, as well as directly controlling your television.