Home theater with cabel

Assuming you've got an extra input on your home theater's receiver, it's pretty simple:
On the back of your cable box (assuming you have a cable box), look for "Audio Out", a pair of red and white connections.
Run a pair of audio cables from there, to an available input on your receiver.  That's all you need to do.
You should turn the actual television volume down, when watching TV through your home theater, as you may get a slight echo from the two audio sources.