Activation lock

If the person you got it from got it from the owner, then they should have their contact information. If they don't, then it's probable that you have a stolen iPhone. Return it and get your money back. If you can't do that, turn it over to the police and let them deal with it.

You ask whoever you got the phone from.

Here is what the previous owner needs to do-->What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support