Find where it comes into your house. Measure the distance. Go to Radio shack buy a spliter, screw in the cable to the spliter and hook it up to the TV. have a beer.
You may want to get a signal booster as well, for the splitter. Install it on the coax before you split the signal. Or if you’re running a lot of coax from the splitter, add it to the split signal that’s going to the HDTV.