Best way to get from Frankfurt airport to Trier, where the tour begins.
Easiest is rail to Koblenz, and from there, change to Trier. Go to the Deutsch Bahn website for more information and for other possible connections.
As Tom mentioned, the bahn.de website has all the information you'll need for that trip. One point to note is that there are two rail stations at the Frankfurt Airport, a Regionalbahnhof (local trains) and a Fernbahnhof (long distance trains). The shortest trips to Trier are via the Fernbahnhof (2H:48M with one change at Koblenz), however those cost a bit more.
As I recall, the route to the rail stations are well marked at the airport, so you shouldn't have any trouble finding them. There's a ticket office there, so you can buy your tickets either from a Kiosk or a staffed ticket desk.
I'm sure you'll have a great time on the G/A/S tour!
My sister did the RS G-A-S tour this past September and chose to fly into Luxemburg City as it was a much shorter train ride to Trier than going through Frankfurt. She was going to have to fly in a day early anyway so she did some sightseeing there before moving onto Trier.