If you reside on the lower floors of apartments or far from the nearest tower, you may have difficulty gathering a strong, steady signal. In this case, you can opt for an outdoor antenna.
Antenna Guide
At Antenna Web, you can check the digital terrestrial signal strength at your location, as well as suitable antennas to meet your needs.
In general, if there are no tall buildings, buildings or obstacles in the vicinity and if the emitting tower is within 30-40 miles, a simple indoor antenna will more than suffice; you should have no problem viewing digital television. It helps if you place your antenna as close to a window as possible.