The standard PIN code can be found on the plastic card, which was holding your SIM card. You preferably change it by a code you only know.
How to change your PIN code? If you want for example the code “1375” as a new PIN code, you enter *04oldPIN13751375# on your phone and press the dial button. The PIN code is now changed to your new code.
For security reasons, you better do not choose sequential numbers (1234) or same numbers (2222).