Depends on the TV. If it has a tuner built it then all you need is an antenna.
If the TV doesn't have a tuner then you'll need a antenna and a tuner.
If you are in the US there are a couple sites that can help you choose an antenna and which direction it should be aimed(if it's directional).
The easiest thing to do would be to check in the "apps" section of your particular box and download the network one associated with whom you want to watch. In some cases there isn't one available.
There are other services that you can check into that "supposedly" offer network support but often are behind for a moment, in other words you aren't watching live.