If the TV doesn't have a digital tuner in it, you will have to get a small box to convert the signal from digital to analog. If you have a newer (2008 and newer) TV that has a digital tuner in it, all you need to do is plug an antenna into it. You will also need some sort of antenna that picks up UHF signals. I have had luck with stripping the shielding off of a piece of coax cable and connecting the other end to the TV. If you have an antenna on a radar arch, it will still work. It doesn't need to be a "digital" antenna. The off-air digital signal is easier to get when compared to the old analog. You either have a great picture, or none at all. You won't get the ghosting or shadows that you used to get with the old analog signals. I hope this helps...
I have a 23" tv with a digital tuner built in. I bought a flat digital antenna at Walmart for like $10. Like Cableguy said, just plug it in and you're good to go. I am able to get 2, 4, 11, 53, 22, 19, 13 all in hi def over antenna.