since you said "plasma" in your title...i'll assume you have a plasma and that you probably went with a cheaper set that is called a "monitor" instead of a tv. hd monitors do NOT have RF Tuners built into the tv and require a box of some type.
if you have (or can get) hd cable, thats the route i would recommend. the rf from your wall will go into a cable box and the cable box will turn that into an HD feed that your tv can take in.
if you are unable to get a cable box, you have two more options.
1.) (Best option) go to a local electronics store and buy an external ATSC tuner. they range from $200+ and are made by companies like samsung, sylvania, lg, sony. this way, you can still get hd without upgrading your service through an antenna and you can still use ur existing cable.
2.) (Worst option) get a VCR. most stero vcr's will let you input a signal through RF and output it through RCA. if you go to an electronics store and talk to someone who doesnt know anything...they'll tell you that you need an RF modulator. they are wrong. you need the OPPOSITE of what an rf modulator does and most vcr's will let you do that.
your last option is to return the tv and get something of better quality. this is what i would recommend, but that is entirely up to you.