Does anyone know...
1) Contact the carrier it is locked to and get the unlock code
2) Download unlocking software. Google Sony Ericsson SIM unlock
3) Third party unlocking companies. Google it.
Go to http://www.davinciteam.us/index.html
I have used them in the past and it is great. Just make sure you read everything over before you start to do anything because you could end up with a paper weight. No as hardcorehavok stated you could call the carrier. That is off course if you are with that carrier and the phone is registered to your account. One option is free and the other will cost you about 20.00 US.