Go to Settings, then choose iCloud, and Documents and Data. Make sure that you turn off Use Cellular Data.
Go to Settings, and choose iTunes & App Store. You will see Cellular Data, toggle it off.
Go to Settings, and tap on Cellular. You will see many applications that can use data. Choose the app, and turn off the data usage.
Backup your data and go to Settings>General>Reset>Erase all Content and Settings. After several minutes, your device will be set up as brand-new.