Travel Note
There are two airports to fly into for Istanbul. The easiest is most likely the main airport, Istanbul Airport. Istanbul Sabiha Gökçen International Airport is fine, as well, but a little further out from the city center.
Travel Note
Here is a link to some average taxi fares for reference:
Travel Note
Here's an overview of transit options from Istanbul Airport including shuttles:
Travel Note
Here's an overview of transit options from Sabiha Airport including shuttles: