Everybody working in the United Kingdom requires a National Insurance Number in order to work, it effectively tracks your tax contributions to ensure you are paying the correct amount.
You must have a passport or visa that makes you eligible to work in the UK.
To apply for a National Insurance Number you will need to call the Job Centre Plus information line
Telephone: 0345 600 0643
Monday to Friday: 8am to 6pm
Note you can only apply for a National Insurance number once you are in the UK.
It is likely they will want to arrange a face to face interview with you to prove your identity.
For more information on how to apply for a National Insurance Number click this link.