(If your TV has an HDMI port, continue on to "Place the Wii U Sensor Bar.")
The Wii U comes with an HDMI cable, but older TVs may not have an HDMI connector. In that case you’ll need a multi out cable. If you have a Wii, the cable you used to connect that to the TV can be used with your Wii U. Otherwise you’ll have to buy a cable.
If the TV accepts component cables (in which case the back of your TV will have three round video ports, colored red, green, and blue, and two audio ports, colored red and white) then you can use a component cable (compare prices). If you don’t see that, then there are hopefully three A/V ports on your TV which are white, red and yellow. In that case, get a multi-out cable that has those three connectors. If your TV only has a coaxial cable connector then you’ll need that three-connector multi-out cable plus the appropriate RF modulator. Alternately, if you have a VCR it probably has an A/V input and coaxial output you can use. Or you could just buy a new TV.
Once you have the appropriate cable, plug the multi-out connector into the Wii U and plug the other connectors into your TV.