First things first what you need to know about traveling in Istanbul, Turkey, is that you need to own an “Istanbulkart”. You cannot pay cash to the shuttles or public buses. Obtain your Istanbulkart upon your arrival at the airport. In case you forget, you can also get it from Sultanahmet and Eminönu. After you register and load your card, you can use all public transport in Istanbul such as public buses, trams, subway, ferries.Istanbulkart's price is 13 TRT(no credit paying). The cost of 1 ride on public transport is 4.03TRY.

NOTE: Istanbulkart doesn't come with credits and needs to load(cash) to use it for transportation. You can load your card with a vending machine, wait for the scan, and then deposit cash into the vending machine, and load credits. After you can use your transport.