How Old is Your TV?
If you are lucky, your TV already has a built in Digital tuner. This depends on how new and pricey your TV is. A TV from 2007 or later probably has a DTV tuner, as do many nicer TVs from 2004 on. Only very nice TVs sold between 1998-2004 stand a chance of having a built-in Digital tuner. If your TV is from 1998 or before, it almost certainly does not have the ability to handle digital on its own.
A Digital ready TV is usually labeled as such, or it will be listed in the manual. If your TV is labeled (or the manual states it supports) “DTV" or “ATSC" you are fine. There are other designations that might be used, according to a FCC factsheet: "These labels or markings may contain the words 'Integrated Digital Tuner' or 'Digital Tuner Built-In.' 'Receiver' may be substituted for 'Tuner,' and 'DTV,' 'ATSC,' or 'HDTV' (high definition television) may be substituted for 'Digital.'"
An easy way to find out if your TV is ready, particularly if your manuals are not handy, is to visit this website. You can enter the TV’s make and model to see if it turns up on their exhaustive (but not quite complete) list. If you still are not sure, go straight to the source and check with the company that made the TV, either online or by phone.
If you already have a tuner in your TV (or all your TVs if you have several), you are probably ready for digital. You may need to adjust your antenna, or in some rare cases, replace it. If your TV can’t handle digital, you will need a converter. In fact you will need one for each TV you have that gets an over the air signal but doesn't have a digital tuner.