How old is your TV? Are you sure it doesn't have a digital (ATSC) tuner?
If it really is "HD ready" like the 32" Panasonic LCD that I bought three years ago, please note that the "coupon box" converters that we discuss in this forum output only standard definition, not HD. They can pick up HD channels, but downconvert them to SD.
If your set is capable of displaying HD, and doesn't have a digital (ATSC) tuner already, you should get an HD tuner like the Samsung DTB-H260F. There's a big thread about it in the "HDTV Technical" forum next door.