How come I cant get my hdtv to show hd quality?

I bought a hitachi plasma 42 inch hdtv and just bought a hd box from suddenlink, but even after connecting everything the way it should be the channels that should broadcast in hd still look pretty bad. Do i need some type of antenna or something? Any help would be much appreciated.

….the elite that comes with an HDMI cable? Do you recommend I get the premium or the elite, I am probably going to get more than 2 games now that i think of it (maybe around 4-5 games) and will be downloading a lot of demos from the marketplace, premium or elite?

HDMI Cable:

http://www.thegamesnews.com/wp-content/uploads/2006/08/hdmi.jpg

  

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