Your National Insurance (NI) number is sent to you in a letter, generally just before your 16th birthday (or on a card if you turned 16 before October 2011).
The number is in the format of 'QQ 12 34 56 A'. If you can't find the letter or card (or a payslip, P60 form or tax return which also display your NI number) and don’t know your NI number, you can ask HM Revenue and Customs to confirm this for you. There are details of how to find your NI number on the GOV.UK website.
Even if you are working part-time or earning a wage too low to pay NI contributions, you must still have a NI number to take a job.