Unlock iPhone

It depends upon how the phone is locked. If it is Passcode Lock, follow these instructions...

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

If it is Activation Lock, check these instructions...

How to remove Activation Lock - Apple Support

If it is Carrier Lock, check these instructions...

How to unlock your iPhone for use with a different carrier - Apple Support

If you're referring to a carrier lock, you need to contact your carrier. Apple does not handle those.

~Lyssa

Lyssa is right if it is your Phone Company's Lock.