Hi suleyman214,
It sounds like your iPhone is locked to the previous owner, with what is called Activation Lock. You can learn more about this on this page:
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
Do you remember when you purchased your iPhone if it was at the "Hello" screen? This would mean that the previous owner followed these directions:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
If you had previously logged into your iPhone with your own Apple ID, then contact the Apple Authorized Service Provider who performed the service. They may have inadvertently left your iPhone signed in with a testing Apple ID.
Otherwise, you'll need to contact the previous owner, so they can remove the iPhone from their Apple ID, as shown above. There is no other way to remove someone else's Apple ID from a device, as this is a security measure.
Take care!