When you are on bahn.de it is a little tricky to make a reservation the first time (the subsequent are easy because you have done it before). The key is to go through like you are going to buy a ticket and at the end there are several radio buttons. click on the one that is "reservation only". Cost is 1.50 euros per person. very cheap and very necessary during the world cup. I was just in Germany for the WC and it was fantastic. The Germans were very friendly and great hosts.