Activation lock

If it's locked to your account and you can't remember it : If you forgot your Apple ID - Apple Support 

Or, and if you bought the phone new (i.e. it’s not second-hand), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the phone from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it.

If the phone is second-hand contact the person you bought it from, Apple won't remove activation lock from second-hand devices.