Here's how to travel between Atatürk Airport and the Büyük Otogar (main bus terminal) in Istanbul, Turkey (map):
Airport to Bus Terminal (Otogar)
Atatürk Airport has its own Metro station beneath the terminals.
Paying the Fare
You should have a minimum TL8 to buy Metro tokens (jeton) from a Jetonmatik machine at the Havalimanı (Airport) Metro station.
There is a machine from which to buy an Istanbulkarttransit pass at the airport Metro station. It may or may not be working.
In theory, the machines accept lira coins and notes/bills. You should be able to get TL1 coins and small notes (TL5, 10, 20) from a currency exchange office or cash machine (ATM) in the baggage claim area. (If you buy something at the duty-free shops in the baggage claim area, or in the shops outside the secure Customs area, you can ask for TL1 coins and/or small TL notes as your change).
There are also currency exchange offices, ATMs and shops in the International Arrivals Hall both inside and outside the secure Customs area. Their rates of exchange are usually not advantageous, so don't change large amount of cash here.
Entering the Metro
When you have your coins, follow the signs toMetro/Subway, go down the escalator (or elevator), through the sliding glass doors, turn right and follow thesigns along the broad underground concourse to the Havalimanı (Airport) Metro station on the left at the end. This concourse beneath the parking garageconnects the Domestic and International terminals.
Pass through the turnstile, descend the escalator (or elevator) and board a Metro train for Aksaray (download a .pdf map of the system). The Havalimanı (Airport) station is the end of this Metro line, so you can board any train—they all go toward Aksaray. You can't get the wrong train.
Stay on the Metro to the Otogar station, the 10th stop. Take the escalator up and you'll be at the heart of Istanbul's Büyük Otogar (Main Intercity Bus Terminal).More...
Bus Terminal to Airport
To travel from the Otogar to Atatürk Airport, look for the Metro signs in the building at the center of the terminal complex. Buy a token (jeton), descend the escalator to the platform, and look for the TV monitor signs with arrows indicating the track on which the Havalimanı(Airport) trains run.
Board the train and take it to the end of the line at the Havalimanı station beneath the airport terminals.
From the station, walk along the underground concourse to either İç Hatlar (Domestic Flights) or Dış Hatlar(International Flights).
You will have to pass through an x-ray and metal-detector security checkpoint to enter either terminal, then another security check to enter the gates area.
—by Tom Brosnahan
Atatürk Airport to S Gökçen Airport