I had to do this last summer. I had a tip from a German friend that ICE tickets are cheaper when you buy them in advance, so a friend and I bought it from the internet on the Deutsche Bahn website (www.bahn.de) . Any credit card will do. Then you have your ICE ticket ready to go, so you do not have to worry about bying it there.
From the airport, you will have to take the S Bahn to the Frankfurt Hauptbahnhof (main station). I believe it will be either the S8 or S9, but you should check that out. The thing I had the most problems with was to buy a ticket for the S-Bahn because I was not familiar with their transportation system.
In Germany, you buy tickets for trains, S Bahns, U Bahns, Trams... from vending machine at every station, and you pay according to where you are and where you want to go. The machine will ask for the number of the destination, Frankfurt is number 50.
You enter 50 and then select einzelfahrt for a one way trip. After paying the asked amount, you will get your ticket.
In the German public transportation, they do not systematically check the tickets (they do in linger distance trains I believe, but not really in S Bahns and even less so in U Bahns, Busses and Trams). However, if you get caught without a ticket, you will have to pay a 40 Euros fine.
In the S Bahn, they announce the next station, so when you hear: nächste hatlestelle, Frankfurt Hauptbahnhof, you know it's time to get out.
At the station, there is a screen indicating on which gate your train will be. It indicates the direction and the ICE number. this will be available I believe 30-60 minutes before your departure.
I think that's all.
been there. done that.