Georgia,
Are you referring to the main Hbf in Frankfurt or the stations at the airport?
As you may know, there are two rail stations at Frankfurt airport, a Regionalbahnhof and a Fernbahnhof. Trains leave from the airport to Berlin from both stations, with a travel time of about 4.5 hours. There are a few direct trains and others with one or two changes. Which station you'll use will depend on which train you can connect with (you may be able to get the 08:51 departure from the Regionalbahnhof, one change at Frankfurt main and then direct to Berlin). All the details are on the bahn.de website.
Although you'll be at the airport anyway, I doubt that a flight to Berlin will save much (if any) time. You'll still have to contend with check-in, security, waiting times, probably re-checking your luggage and then transport from the airport in Berlin. Using the train would (IMO) be a more relaxing and less stressful option.
I'd suggest just buying your tickets when you arrive. There are ticket offices at the airport rail stations and also Kiosks, so it's not hard to get tickets.