First check your phone to see if it is unlocked. How? Insert a different carriers sim card into your phone, power on handset on if the phone allows you to make calls out, ect then your phone is already unlocked.

To unlock: Insert carriers sim card
Power on handset
Wait for phone to fully power up
Enter unlock code( you have to get this from your carrier; some have specific times only they will give this to you like if you plan to travel internationally or go to another provider or if you paid full price for your phone) then press okay. Handset is unlocked.