You can fly to Ataturk Airport (IST), on European Side of the city; and use metro or buses to get to the city center.
Or you can fly to Sabiha Gökçen International Airport (SAW), on Asian Side of the city; and use metro and metrobus to get to the city center.
Coming to Istanbul
You can check these websites for flight tickets:
Transportation in the City
from Taksim to Yenikapı
You can take the metro (M2 line) from Taksim square, Yenikapı is 4 metro stops away.
You can take a taxi to the venue, Gar Music Hall, it will cost about 3-4€ from Taksim.
from Taksim to Kadıköy
You can use shared taxis (or as we call them: sarı dolmuş) between Kadıköy and Taksim. You can find them in front of AKM in Taksim square, and near Metro station in Kadıköy Square.
from Kadıköy to Yenikapı
You can take the metro (M4 line) from Kadıköy square, and change to Marmaray on the first stop to get to Yenikapı.