Someone sold me a iPhone 6s and it's like locked I already rested it too make it brand new but we need the persons apple id what do I do too unlock it complete.

You will need the previous owner to get a proof of purchase receipt and take it to Apple to have them to clear it. That's what I had to do with my daughter's phone.

If you have the proper paperwork proving that you bought the phone from the verified owner, you can take the phone and your paperwork to Apple University and they can help you establish a new Apple ID. However, if you don't have proper paperwork, you would have to have the original person's Apple ID number to get it unlocked. Then you could change the Apple ID and password to a new one for you