It all depends on what type of tv and computer you have. You can use either a VGA cable or HDMI cable to connect to your tv. It all depends on what output your computer has and what inputs your tv has. If possible you want to use HDMI ... Much better picture.
I don't have an Apple tv but from what I've read doesn't it have an HDMI out? If its the newest one I think it does. An HDMI cable should do the trick. But someone on here who has one may be able to confirm that ...