Molly, read this page of the direct.gov website, it tells you how to get your NI number or how to find out what it is. It should have been sent to you a year ago, just before your 16th birthday.
http://www.direct.gov...oBenefits/DG_10014073
It looks something like this: A-01-10-78-D.
You are right, you need to know what it is so they may send you a duplicate letter confirming it, but you need to contact them a.s.a.p.
Scroll right down the page and there is a link to form CA5403 to apply for this information.
I hope this helps.