When hooking up appletv, what if I have a cable box? Hdmi cord goes from cable box to tv.

You don't plug the cable box into the AppleTV. Most TVs have more than one HDMI port, so you plug the cable box HDMI directly into the TV, and then plug the AppleTV directly into the TV too.

If your TV only has one HDMI port, then you have a problem. First check to see if your cable box has two HDMI ports (unlikely, but worth looking)- it might have an "HDMI In" and an "HDMI Out" - if you're lucky enough to have that, then you plug the AppleTV into the "HDMI In" and then it will "pass through" the cable box.

But more likely you'll need to buy an HDMI switch. You can get a cheap one for about $20, or go the expensive route and get a high-end stereo receiver with A/V switching and multiple HDMI ports (then everything gets plugged into the receiver, and the receiver is the only thing that plugs into the TV).

YYou'll want to plug the Apple TV HDMI out to a new HDMI input on your TV, just like your cable box is setup. You change HDMI inputs on the TV from Cable to Apple TV