Oh, if you have a cable box, Keep this in mind.
For HDTV you need at least one of the following connections on the back of your cable box.
1) Component (three RCA jacks, red, green & blue)
2) HDMI (a small, rectangular jack that looks like a squashed USB port)
3) DVI (looks like a tiny egg crate, but with more holes)
If you don't have at least one of these, then your cable box is not able to provide HDTV. Time to call 'em and see what it takes.
If you have these, then you might have a HD box. You must use one of these to get an HD signal to your TV. If you're using one these and you still can't get HD, call your cable company to see what the problem is. You may have a setup issue or you might need to subscribe to a different level of service, generally more expensive.
good luck...