Make sure you have an antenna that can receive a strong signal from the stations you want to see. HDTV and digital television in general is different than old analog signals -- you either get a signal or you don't.
When analog TV was the standard, it was sometimes OK to get a weak signal because you might get a little snow on the screen but could still watch television. That has changed -- a weak HDTV signal will result in no picture at all. It's either on or off.
A good roof antenna and signal amplifier can take care of the "weak signal" problem. Those items don't cost a whole lot, either.