What to Eat
Barduni Zenwich (Zenwich-berlin.de) has a stand just outside the Hackescher Markt station where the other food vendors are. He sells grilled sandwiches with intense ingredients like olive paste and mango chutney. If you're sick of doner kebabs and pizza, this is a good, healthy alternative. The Potsdam sandwich was my personal favorite. Friendly owner. Sandwiches are between 2-3 euros.
In Berlin, you buy your metro ticket OUTSIDE the train, not on it. Be sure to purchase one before hopping onto any train. The fine for riding without a ticket is steep, a pocket oozing 40 euros. The Bahn ticket nazis aren't very nice either, so best to avoid having to interact with them at all.
At the Hauptbahnhof, you'll be able to reserve your seat for future international travel (regardless of whether the departure city is Berlin). The DB Bahn Reisezentrum office on the first floor of the station (above the arrival platform) will take care of this for you. They take cash and credit cards.
Circus Hostel outdoes itself with this travel wiki maintained by staff. If you're not sure what to do, check out their suggestions at Circus-wiki.com/Cwiki. It's pretty thorough, including everything from info on bars to restos, sights/culture to parks, and there is even a section on gay Berlin.