Rick, that is a great post!
Let me add a couple of things:
Rick is assuming - and I completely agree - that you start with a visit to
www.antennaweb.org
where you can enter your address and get a list of channels you might be able to receive. Antennaweb is not perfect! You may not get all your digital stations listed. Go through the list of stations that antennaweb gives you. If a station has the suffix -DT that's a digital station. If you see non-digital stations without a digital equivalent (for example WFAA) , you need to find the channel allocation for the corresponding digital station (WFAA-DT). You can usually find this by googling WFAA-DT, going to the analog station's web site or by going to the FCC's web site. One warning. Some of the stations have elected to move back to their VHF channel, or to a different UHF channel, after the transition to digital is complete. The FCC web site has a list of these "elections".
When you have completed your list of stations by adding the "missing" ones and correcting the channel numbers for any "elections", you
need to decide if you want to get the analog stations as well as the digitals, between now and the shut-off of analog stations in 1999.
Why might you want the analog channels? If you have TVs in the house with only analog capability and you want them to get OTA channels....
Why do you need to make that decision? Because several of the analog channels maybe on VHF; it's likely that your digital channels will all be on UHF (or maybe hi-band VHF) and antennas for UHF (some of which perform well at hi-band VHF also) are significantly smaller than the VHF/UHF combos you need for distant reception. Life is much easier in general if you don't have to deal with lo-band VHF stations.
So now you have a list of stations you want to receive, with their corresponding ratings from antennaweb (where available). Now you can use Rick's post to help you find the correct antenna.