Make your visa application in person and one month before you proceed to Turkey to avoid any inconvenience.

Sticker-type entry visas: Tourists and business visitors from the following countries do require visas and can obtain a sticker-type entry visa at the point of entry for a fee (prices are dependent on nationality , see below):

Australia, Austria, Belgium, Canada, Ireland, Italy, Malta, The Netherlands, Portugal, Slovenia, Spain, the UK* and USA for stays not exceeding 90 days;
Greek Cypriot Administrative Region, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovak Republic for stays not exceeding 30 days.
Except   Nationals of Bulgaria who do not require a visa for stays up to 90 days, but do require a visa for transit and for entry at certain points in Turkey; check with consulate.
Citizens of the following countries can get visa at the point of entry into Turkey for a fee:

Valid for 3 (three) months

Australia (cost: $20 / €15)
Canada (cost: €45)
Ireland (cost: €10)
US (cost: US$20 /€15)
UK (cost: US$20 / €15 / £10)
Portugal (cost: €10)
Italy (cost: €10)
The Netherlands (cost: €10)

Valid for 2 (two) months

Ukraine (cost: US$20)

Valid for 1 (one) month

Azerbaijan, Moldova, Russia (cost: €10 or US$20)
Norway (cost: €20)
Slovakia (cost: €10 or US$15)
Serbia (cost: €10)
The citizens of the countries/territories listed below can enter Turkey visa-free for 90 days unless otherwise stated:
Andorra, Argentina, Bahrain, Bolivia, Bosnia-Herzegovina (60 days), Brazil (30 days), Bulgaria, Chile, Costa Rica (30 days), Croatia, Czech Republic, Northern Cyprus (Turkish republic of), Denmark, Ecuador, El Salvador, Estonia, Finland, France, Georgia, Germany, Greece, Guatemala, Honduras, Iceland, Hong Kong, Iran, Israel, Italy, Japan, Kazakhstan (30 days), Kyrgyzstan (30 days), Korea (South), Latvia (30 days), Liechtenstein, Luxembourg, Macau (30 days), Macedonia (60 days), Malaysia, Moldova (30 days), Monaco, Mongolia (30 days), Montenegro (60 days), New Zealand, Nicaragua, Paraguay, San Marino, Singapore, Sweden, Switzerland, Tajikistan (30 days), Trinidad & Tobago, Tunisia, Turkmenistan (30 days), UAE, Uruguay, Uzbekistan, Vatican City and Venezuela.
German citizens don’t need a visa for stays up to 90 days and can even enter with their national ID card (Personalausweis) or an expired passport/ID unless arriving at the non-Council of Europe land border crossings (i.e. from Iran, Iraq and Syria). 1

For Study – Work

The above information is for tourists. If you are going to study or work in Turkey, you must obtain appropriate visa from Turkish diplomatic/consular missions, prior to proceeding to Turkey.
More information can be found at the Turkish Ministry of Foreign Affairs website