When buying a SIM-card it was attached to a rectangular plastic plate attached to the envelope. The remains of the plate to throw away is not necessary. It is located on the strip and around the strip words: PIN1, PIN2, PUK1, PUK2. RUB the strip with the edge of a coin or a fingernail next to PIN1. Under the top layer will be the combination of four digits. This is the pin.
If the map pin is not set, believe about the rest of the documents included with the SIM card.
If the SIM card is not purchased by you, or if you just documents stored in another person, check them out. Find and replace all codes new keep it in a safe, secure place.
If the pin code was not found, try to use combinations that are often put in the default pin-code: 0000, 1234 or the like.