When you buy a mobile phone, you will actually be buying two things: the mobile phone itself, and a Subscriber Identity Module (SIM) card.
Mobile phone shops are practically everywhere in Turkish cities, from posh stores of the major mobile services providers to street stalls, where you can buy used mobile phones.
Once you find the model you want, you will need a SIM card from one of Turkey's mobile service providers. A SIM card is the brains of the phone. Besides giving the phone a Turkish phone number, it stores information about which service to connect to, your user preferences, and the telephone numbers of your contacts.
To buy a SIM card, go to one of the major network provider's stores. Take your passport with you. You will probably be shown several SIM card kits (in standard envelopes) with telephone numbers printed on the outside. You can choose whichever telephone number you like from those they have available. The clerk will then take your payment and passport to register the SIM card in your name. Then he will install the SIM card in to your mobile phone.
The prices of these SIM card kits, even from the same provider, can vary from place to place, so shop around for the best price. Prices will also vary depending on how much calling credit is included with the kit.
If you have a mobile phone from another country, you can also use that with a Turkish SIM card, for 60 days. If you want to use it longer than that, you must pay a tax and register it.