Since you'll be in Germany for 7 days, I would suggest buying a 7 day pass as you'll be using some sort of transport to get around so, I reckon, this would be most cost effective for you. U-bahn and S-bahn are the quickest way to get around within the
city.
You can buy 7 day passes from S-Bahn ticket machines, S-Bahn Berlin customer centers and ticket counters at S-Bahn stations.
Prices for 7 day pass:
7 days ticket Berlin zones AB = €26,20
7 days ticket Berlin zones ABC = €32,30
Prices for single tickets are as follows:
Zones AB €6.10
Zones ABC €6.50
Hope this helps
Vacationbuddy