How do I get my N.I. number? I'm a UK national that moved abroad at 8 and never received one

I am a UK citizen--I was born in England, but I moved to America when I was 8 years old and never received my national insurance number. I'm now 22, want to move back to England to work, and need my N.I. number to do so. How do I get one? Do I need to apply? Or just request to have one sent to me?

Hi

You can start working without a National Insurance number, however, you may already hold one.  
You can complete and send the CA5403, which wwe can use to trace your national insurance number.  
If you do not have one, once you return to the UK you can apply by going to Apply for a National Insurance number

Thanks