http://www.bahn.de/international/view/en/index.shtml is the English language site of the German railroad company. I'm afraid it's rather weird and unintuitive.
I keep getting different results for that putative Erfurt - Berlin round trip of yours
According to the information I gleaned, the Erfurt - Berlin round trip ticket price fluctuates between 78 and and 108 Euros (please don't ask me why!
). The trip takes about 2 1/2 - 3 hours. Some trains are direct, or you have to change trains once.
My ultimate advice would be to see a travel agent (although many of them have quit dealing with the German railroad company and may no longer be of help) or to go directly to the railroad station, conveniently and unlike most airports located right downtown, and to head for the travel center. You can pick up printed train schedules for lots of destinations there, and the people behind the counters tend to be knowledgeable and will give you good and current advice.
I see you'll be going to study in Erfurt "next year." That's quite a while off, but good for you to start early with your planning! Do bear in mind that those prices may change.