From the airport, many package holidays to Berlin already include a transfer, but there are plenty of taxis available as well as trains on hand to get you into the centre of the city. Buses run throughout the day and night, and you can pick them up pretty much anywhere. The city centre is actually quite compact, and walking is usually adequate to get you between the main attractions, although if you're short on time, and trying to pack a lot into your trip, then taxis are widely available.
Hiring a car is a good idea in Germany if you plan to leave the city, and tour the surrounding area, and the roads around Germany's showcase city are excellent, and well signposted. You should be aware that certain sections of the Autobahn are not subject to speed limits, and it is not unusual for drivers to get a bit silly, and push their cars to the limit.
