Step 1: Get the IMEI number by typing *#06# on your phone and hit the sending button. You will use the number to get the unlock code.
Step 2: Ask your carrier to get the unlock code, they will ask you for the IMEI number which you have just got in previous step.
Note: some carriers provide you the unlock code for free but other won?t give you. If you can?t get it from the carrier, you should find a local store to get the unlock code. If you can?t find one, I suggest you to search online. I found the cheapest on for $11.90 only for the unlock code. Visit here: http://www.unlockl-any-lg.com
Step 3: Now turn on the LG phone with the new sim card.
Step 4: The phone will ask you to enter the unlock code and you can now enter the unlock code to unlock.