If you visited Turkey in past, there is a big chance that you took your visa at the border/ airport, but if you plan to come again, you should know that there it will be different this time. From April 10th 2014 visa on arrival will not be available anymore, and you should apply for an electronic visa through the official portal of the Ministry of Foreign Affairs of Turkey. To get more detailed informations and apply for your visa, please visit: https://www.evisa.gov.tr/en/