If you come in via the airport, you can purchase multi-day discount books with transportation. These give you unlimited access to the subway, train to and from airport, buses as well as discount tickets, including tours and museums. It is well worth purchasing. The city is walkable, but huge (like Manhattan). The subway is very good and safe and you may find it helpful to use.

You didn't ask about hotels, but if you have any questions, do not hesitate to ask. Food is good and well priced. Try to stick to the East Berlin side. This is the historic area. West Berlin looks like Fifth Avenue in NYC. It was very nice for high end shopping and there are nice hotels, but there is little of historic interest there and it takes a fairly long time to get over to historic and museum areas.