If this is a used device, you will need to contact the previous owner to have them log out of iCloud or go to iCloud.com/Find My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless.
Activation Lock - Turn off Find My iPhone
If necessary, if you are the original owner, take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store.
Genius Bar Reservation US
or contact Apple ID Support. They will need to see the original purchase receipt/paperwork also.
Apple ID Support
Apple ID Support (2)
Apple Support by e-mail or chat
Apple Support using Twitter
Or fill out and submit this form.