The TV service provider will determine what you may have to buy - maybe nothing. Around here, Comcast usually gives cables with their required settop boxes, including HDMI if the box has it.

Comcast is on a program to require all users to use settop boxes, and encrypting all but the local stations and most basic service level. Only the most basic service level allows direct connection of the cable to the TV, and you are limited to the local channels and a few more. Around here, that's channels 2-29 & 75-99. They may be a combination of analog, digital and HD (locals only). For the next step of digital service (non-HD), they give a DTA ("digital transport adapter") that deencrypts your standard digital non-HD channels onto RF channel 3 or 4, and you set your TV to channel 3 or 4, about the lowest quality way to do it. If you want to see the HD on the cable, you have to rig up a switch to select direct cable or DTA into your TV.

HD/DVR/HDDVR subscribers get a more advanced settop box, usually with several output options. Some older boxes don't have HDMI, so if you want the best quality available and your TV has it, ask for a box with HDMI. Some older boxes have an older digital interconnect, DVI, which can be adapted to HDMI. Analog Component works very well, but does have limitations. S-Video and composite are non-HD and are a last resort nowadays.

+1 on using Monoprice for cables.