how do i locate the previous owner if i don't know him/her?

If you are trying to activate an iPad or iPhone and it is asking for the original owners Apple ID and password, you have encountered the Activation Lock. This is a security feature that prevents thieves from setting up and using a stolen or lost iPad or iPhone. You have no alternative. No must contact the original owner to get permission to use the device. If you cannot contact the original owner return the device to where you bought it and get a refund. You will never be able to activate the device and no one can help you do it.

Apple will absolutely not contact the original owner for you, and there's no other way to identify them for security reasons. The phone is useless.