how do i use internet cable wire to watch cable tv??

i have cable internet service which i use only for my computer but i was recently thinking, why not have tv and internet from one wire..when i tryed putting the cable wire which usually goes to my cable box into the back of my tv, i got cable channels but they weren't clear, i could see very little of what was on tv but not clearly at all.Do i have to change something to make it clear b/c one of my friends has done this with a splitter and its very clear on his tv.Any help would be appreciated