Do you have a ATSC tuner?
Your TV has an HD tuner built in, but you have to feed it a signal from something. If you don't get the HD package with Dish, or if Dish doesn't have all of your local OTA channels, you can try a set of HDTV "rabbit ears." If you live in a marginal signal area, you'd be better off with an outside antenna -- they make them that mount on your dish and feed the OTA signal to your Dish receiver. Or you can install a separate one and run another coax line to your TV.