Depends what you want to do ?
:- Watching TV - you can buy a HD decoder of your choice and depending which company you buy from you get the choice and number of HD channels. Like Dish Tv may have 4 on the other hand DSTV may have more
:- Watching Movies - This you can achieve in several ways, but first you have to get HD contents, either download it or buy it. Now delivery to TV can be via HDMI or DVI. you can use HDMI on your PC or laptop, HD player, or media content player which has internal or external HDD. Try to search for Western Digital Media player, Popcorn Hour, Apple Player