The budget airlines (Ryanair, Easyjet) use Schönefeld airport, in the south-east. Take the S-Bahn into the centre of town, or the Airport Express from the same station. It’s cheap, fast, and - since this is Germany - punctual. Don’t take a taxi from Schönefeld airport to the city centre: it’s a long way.
Some airlines (like Air Berlin) arrive at Tegel airport. From here take the X9 bus to Zoologischer Garten.
Send your feedback or queries to firstname.lastname@example.org