Frankfurt Airport has 2 Terminals, Terminal 2 and Terminal 1 which are connected via a tramway (Sky Line)which takes about 3 minutes each way. Look for signs directing you to each terminal.
Terminal 1 (The bigger of the two Terminals) has an underground walkway to the Airport train station (Not the Main Haupt Bahnhof). From that station you can get on trains that will take you to the main train station (Haupt Bahnhof) or directly to your destination.
You can also walk the bridge over to the Business center and then go down to the lower level where the train station is.
The travel center offers live personnel who will assist you with buying tickets. There are also automated ticket machines near the travel center and near the tracks.
Several people have told me the people at the travel center did not speak English. So make sure you have something on paper you can show them so they understand where you want to go. Just in case.
Many trains will go first to the main train station and then to their destination.
Remember ICE (Inter City Express) is the way to go if time is important. Cheaper trains will stop at all stations and mean 2-4 times the length of travel time.