The type of Antenna you need depends on how close you live to the towers. Outdoor antennas cost about $300, but Indoor Antennas are cheaper for those living within 15 miles, or so, of towers.
Antennas are CHEAP because they've been around a long time and you don't need a special HDTV Antenna to receive HD signals perfect, free and forever. Simple Antenna basics are listed below by Pennsylvania City...
Pittsburgh Erie Harrisburg
...other U.S. Cities and States