Turkey demands an official entry visa from the citizens of some countries. Before your departure to Turkey it is better to check with a Turkish Consulate in your country to determine if you need a visa. In some cases, you can obtain a visa upon arrival at Ataturk Airport.
The procedure for different countries is outlined on the following web site for each country. More details are available in the following link:
You can also locate the Turkish Consular services in your country by visiting the Visa Process page of the Consular Website of the Republic of Turkey.