If I am arriving from a U.S. flight into Frankfurt and then catching a train directly from the airport, which station should I use when querying the www.bahn.de site - Regionalbf or Fernbf? There's no explanation on the railway site.
Also, can someone please give me some step-by-step instructions on how to buy a railway ticket at the station and get on the train platform? I'm usually jet-lagged and stupid with fatigue at the end of a long flight, so the more idiot-proofing I can acquire, the better! Thank you.

It's easy to do. there is great signage. Just follow the signs and you'll be at a ticket window where someone speaks English. Then just follow some more signs.

Thanks julies, that's reassuring.
If I'm doing research beforehand on the timetables though, which name (of the train station at the airport) should I choose? Thanks.

Do you know which terminal you're coming into/out of? When you type in Flughaven, it asks for which terminal (along with several Tors.) I was there in Nov and saw the DB as I walked out but didn't pay attention to which Terminal I was in.

The Fernbahnhof is for the long-distance trains and the Regionalbahnhof for the S-Bahn and other local trains. But on www.bahn.de you can just put FRA into the query and get both of them.

The Regionalbahnhof is downstairs in the main terminal. It sends off mostly regional trains to local destinations. The Fernbahnhof is for faster long distance trains and requires a bit more walking from the main terminal; follow the signs. I''ve never left from the Fernbahnhof myself. At the Regionalbahnhof, there are ticket machines and a DB office where you can stand in line and purchase a ticket. The red-and-blue DB machines have an English option; you can check schedules and purchase tickets with a credit card by using the touch screen. There are also RMV machines which dispense tickets for certain nearby destinations (you punch in a numerical code for some destination, specify your age and class of ticket by pressing other buttons, insert your money.) These machines are a little tricky if you don't do German.)

You'll need to look at the posted schedule (goldenrod), the electronic destination board in the station, or your printed itinerary from the DB machine (or DB counter) to determine which track you leave from. Signage in the stations is general good ("Gleis" is the word for platform/track.)