how do you unlock an iPhone?

What do you mean by 'lock'? It can have several meanings. If it's locked to a particular carrier (service provider) you will have to take it up with them - only they can release it so you can use a SIM from another carrier.

Or it can be locked with a passcode - four or six numbers - which you've forgotten. If that's the case you will have to connect it to iTunes and erase it: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Or does your screen look something like this (with the ID hint being not yours)? -

Did you buy the device second-hand (used)? If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it).

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.

If on the other hand you bought it new, and the ID it's asking for is yours but you've forgotten it, go to https://iforgot.apple.com to retrieve the ID and reset the password. See also If you forgot your Apple ID - Apple Support

Failing that, take the device, the original purchase documents, and some ID to an actual Apple Store (make an appointment) and ask if they can unlock it.

You call the cellular service provider that the phone is currently on and then you ask them. Your iPhone can only be unlocked by the service provider based on their guidelines.

Axel F.