What you do is wait until you're home alone or late at night when everyone else has gone to bed. Then grab the remote and program your TV's sleep timer to come on at 3:00 a.m. or any other time where everyone else is going to be dead asleep.
If you're doing this before you go to bed, turn the volume up as high as it will go, put the TV onto a channel with either a lot of snow, or, if you have digital cable, music channel 913(Metal) will do just fine and then turn the TV off. If you're programming it while home alone, then put the TV back the way it was and adjust the channels and volume before you go to bed.
Later, at the time you specified, the TV will pop on at top volume and scare the crap out of your parents!
This prank is also great for slumber parties where everyone is asleep in the living room. You can also do it to an enemy's TV and get him/her in trouble for making so much noise. *evil grin*