What carrier?  If it's Tmobile follow the instructions in the link below.

https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

If your phone is from another carrier.  You must meet the other carriers SIM unlock requirements to have the device SIM unlocked by them.  T-Mobile can only SIM unlock devices SIM locked to T-Mobile USA.  For T-Mobile to SIM unlock your T-Mobile USA SIM locked device, you must meet all of the SIM Unlock Policy requirements at the link gramps28 posted as a customer of T-Mobile to qualify to have the device SIM unlocked.