Turkcell GPRS Settings (Android)
First of all make sure data over mobile network is enabled.
Settings > Wireless & networks > Mobile networks > Data enabled = true
Make sure you set the following values. (case sensitive)
Settings > Wireless & networks > Mobile networks > Access Point Names > New APN
Name: Turkcell Internet
APN type: default
Once you save and switch back to home screen, 3G icon should appear at the top next to signal strength bars.