All legally employed people in the UK need to have a National Insurance [NI] number. Foreign people coming to the UK who have the right to work can 'create' a temporary NI number or obtain an 'official' one. When looking for a job most employers will request a NI number as a condition of working.

TEMPORARY NATIONAL INSURANCE NUMBER
This is created as follows:
'TN' + date of birth [6 digits] + M [Male] or F [Female]
Example: for a girl born on 10th June 1982 = TN 10 06 82 F
Important: a temporary NI number is valid for 12 months only. Before 12 months you need to obtain an official NI number

OFFICIAL NATIONAL INSURANCE NUMBER
This can be obtained from a Job Centre or a DHSS (Social Sceurity office) in the area where you live in the UK.
Important: obtaining an official NI number is a relatively long process and is only worth doing is you plan to stay in the UK for a relatively long period.