Berlin has two airports, but most international flights arrive into the Berlin Tegal Airport. The public transport system, particularly the train network, is easy to use. For private trips to Sachsenhausen, take a train to Oranienburg (about 30 minutes). From the train station, it is a 20 minute walk to the camp, or you can take the #804 bus.
Sandemans New Europe runs free walking tours in the city, as well as other tours including the day trip to Sachsenhausen. Most walking tour groups start from the Brandenburg Gate.