Only the carrier can unlock your iPhone. Verizon's iPhone 5 is already unlocked and can be used with any GSM provider. Sprint unlocks if you have been a customer for at least 90 days and only for international SIM cards. AT&T - see these tips on how to unlock At&t iPhone 5: digitaljournal.com/blog/21272
If you have another provider, simply call them and ask how to get it unlocked.