Hello and good night to you.
I am sorry to hear that your phone is asking for a PUK number. PUK simply stands for Personal User Code. The phone only asks for this code when someone enters their PIN incorrectly a certain number of times. Generally, there are two PUK codes that each SIM card comes with, namely PUK1 and PUK2. Based on your statement, I assume that the phone is asking for the PUK1 code. This code is usually located on the card (same size as a credit card) that the SIM card was a part of before being pushed out and put in the phone. The other location of the PUK codes would be on the written contract that you signed with your cellphone service provider. In most cases the number would be located at the bottom of the contract.
What I would like to know is:
(1) Did you get the rest of the card that SIM card came with? If so, look on it and you should see the PUK codes there.
(2) Did you get a written service contract from your cellphone service provider that you can look at to see if the PUK codes are on it?
If your answer to both questions is no, then you will have to get in touch with your service provide so that they can assist you in recovering your PUK codes.