Q: Adding new device

I suggest that you ask the person that you purchased it from. No one here, nor Apple can assist you in locating the original owner. If the person that you purchased it from cannot, or will not assist you, then return the device for a full refund. You will never be able to activate it without the original owner removing it from their iCloud account. It is useless to you. You cannot get past Activation Lock, and you cannot add this phone to your account.

I suggest you return it for a refund if you not know the previous owner's Apple ID & password.

Activation Lock cannot be bypass. It will be useless as a phone to you.

It isn't possible. There is nothing to understand. It cannot be done. No one can bypass Activation Lock. Don't believe everything you see on the internet.

OK just got a new IPhone 6s 3D I don't really know but for a great price

What sounds like too good to be true is probably is.

Arm yourself on what the seller has to do for the next used iPhone buy:

What to do before selling or giving away your iPhone, iPad, or iPod touch - Apple Support

Do you really feel that is the ethical thing to do. Just because someone else screwed you by selling you a locked phone, you want to do the same thing to someone else, just because you want to sell it for less? They aren't going to be able to use it either. No one can.