Rick Steves has a great packing list; you can alter it to fit your particular situation. The old adage about "figure out what you want to pack, then take half the clothes and twice the money," is true. No one ever wishes they had overpacked!
http://www.ricksteves.com/plan/tips/packlist.htm
You would probably also enjoy reading his tip board for Germany, as well as the Berlin board on Trip Advisor. (google: Berlin trip advisor forum). The main Germany board would be helpful as well.
If you haven't already, get a German phrase book. Most people in Berlin likely speak some English, but it's polite to at least be able to say "Good day," "please," "thank you," and "do you speak English?" in German. Other helpful phrases are "How much does this cost?" and "Where is the bathroom?"
Be sure you have food and water for the flight. You will get a meal, but it is a long time to be on a plane and it's good to have your own stuff, just in case. We take empty Camelbak bottles and fill them after security. You can also buy water after security, but a water bottle is a good thing to have with you anyway. You will get hot and thirsty while out sightseeing. You can refill it from public drinking fountains.
Make sure you notify your credit card and ATM banks that you are traveling. Also, take their fraud department's toll free number with you. We DID notify our bank and our card still got frozen for suspected fraud. Once we called, it got sorted out, but if we hadn't had the number, I don't know what we would have done.
Get your cash from ATMs once you arrive.
Have a great time!