It's very unlikely you need a converter box. Those are for TVs that do not have a digital tuner, and unless you got a very early HDTV, pretty much all HDTVs have digital tuners.
Your location is probably the culprit. Use a site like
this one to determine signal strengths at your address. The strength will be color coded and based on the color, you can select an antenna to match (the antenna boxes are usually labeled with what color zone they are good for). The map will also show you the location of the transmitter for each station so you can aim you antenna in the generally correct direction.