The abslolute Public Transportation Card Istanbul is the Istanbul Card!
An electronic, prepaid and rechargeable card, which you can use it for all transportations means in the city, such as buses, metro, ferries, tram, funicular, cable cars and metro buses!
If you come with a friend, no need to buy two of them. The card is anonymous so you can use it for other people too. The initial costs of the card is 10TL and you can find it at many transit stops, such as such as Eminonu, Karakoy, Sultanahmet, Besiktas and of course Taksim metro station.
You can load/reload the card, both at newsstands/small shops (there should be a sign Akbil / Dolum) and at the self service machines, located at the major transit stops. Machines (biletmatik) accept only paper money.
At the self service machines, put the card at the white vertical rectangle, as shown at the picture. If you loose the card, buy another one. There is nothing you can do! This is why you should not overload your card with much money.
The fare of the route is 2.30TL and if you use the card again within an hour, the next fare will be with discount (only for the one who hits first the card). If you want to see your balance, check the small screens above the electronic reader, when you hit the card.
In case you travel to city only for a couple of days and you dont want to buy the card, then you can buy Tokens for your transportation. Each one costs 4TL, you can use it for one route and you can buy them from Jetonmatiks at the major transit stations.