How do I unlock my SIM?

Use a SIM PIN for your iPhone or iPad - Apple Support

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

Are you referring to a PIN code or PUK code? If you have forgotten one of those, contact your carrier. Guessing a wrong PIN or PUK code could cause the SIM to become invalid, and then you would be forced to replace it (which would also involve contacting your carrier).

Or are you referring to a carrier lock? If your phone is locked to service from a particular carrier, only that carrier can release the lock – if they choose to do so. Your chances of getting the lock removed are best if you are (a) a current customer, who (b) doesn't owe them payments on any contract, or is willing to pay off remaining ones.

What kind of lock on your SIM are you referring to? Most likely you will need to contact your carrier, or another carrier.