How do I get a train ticket to Koeln from Frankford International Airport?

I'm flying into Frankfurt and I need to get to Koeln. I know there is a train stop in the airport but I'm not sure where to go for a ticket and what train to take. Thanks!
bleedingpenguins' answer is not bad but contains some mistakes.
There are TWO train stations in Frankfurt Airport: one for local trains ("Regionalbahnhof" - which is below Terminal B), and one for long distance trains ("Fernbahnhof" - 5min walking distance from Terminal B); the signs will lead you to either of those stations.

Buying tickets:
- from a ticket machine at any train station: they also offer an english version; you can use credit cards (at least VISA and MasterCard are accepted), they tell you about ANY connection (fast or slow, expensive or less expensive)
- there are DeutscheBahn counters at each of those train stations. Because Frankfurt Airport is international I'd expect the employees to be fluent in english
- on weekends there's a special offer called "Sch?nes Wochenende Ticket" (nice weekend ticket): for 33,- EUR you can take local trains all over Germany for the whole day, valid for up to 5 persons
- at
you can book a ticket in advance. I never got a seat reservation so far: because trains are running frequently there are usually seats available at any train; and your plane can be delayed so you might miss your reserved connection.

At the provided link you can also get a train schedule for either fast or slower trains. If you're not in a hurry I strongly recommend not taking the fastest ICE train (passing mostly tunnels) but going via Mainz (IC/EC or local train). This will lead you through the very scenic Rhine Valley. That's probably the most beautiful train route in Germany - maybe the most beautiful worldwide...