How to get a national insurance number
Things You'll Need
Telephone the Jobcentre's dedicated National Insurance Allocation helpline. Arrange an 'Evidence of Identity' interview. If you live in the UK and receive Child Benefit, your child will be automatically assigned a National Insurance number and sent an NI numbercard shortly before their 16th birthday.
Attend your Evidence of Identity interview, where you will be expected to prove who you are and that you have a right to live and work in the UK. The interview will be one-to-one but you'll be allowed to take an interpreter along if you need one.
Be prepared to answer questions about who you are and why you want a National Insurance number during the lengthy interview. It's easier if you already have a job offer, but if you are still looking for work you can take proof that you are actively job hunting instead, including rejection letters, copies of applications and job interview invitations.
Remember to take as many official identity documents as you can to the interview to make proving your identity much easier. When your interview date is arranged, you will be told which documents you will need to bring (it differs depending on what you need the number
for). Don't be alarmed if the interviewer requests to keep the documents for a while.
Bring only the real things; no photocopies are allowed. Recent entrants to the country and asylum seekers will also be required to supply travel documents giving information about how and when they arrived in the country.
Don't abandon hope if you don't have any of documents; you must still go to the interview and may be able to prove your identity with the information you can provide when questioned. UK nationals can request copies of their birth or marriage certificates by contacting the General Register Office.
Wait about three to four weeks to receive your National Insurance number in writing if your application is successful and you have satisfied the NI Allocation Service that you are eligible.
Tell your employer your National Insurance number as soon as you start work. Your company will not be able to enter you onto their payroll system unless you provide them with your NI number.
Contact your nearest Jobcentre Plus or HM Revenues & Customs office if you lose your National Insurance Card. Look after your card because you will only be allowed one replacement card during your lifetime.Source: ehow.com