If the phone has activation lock you will have to use the id of the account that it's locked to, you can't link it to another account until after it's been removed from the existing one.
If you can't remember the account's password : If you forgot your Apple ID password - Apple Support
Or, , and if you were the original purchaser of the phone (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