All employees pay National Insurance contributions and if you do want to work then you must apply for a National Insurance number. The National Insurance Number (NINO) is a number unique to you to which will help your employer to pay your National Insurance contributions. No one else can use this number.
Call Job Centre Plus on 0800 141 2075 to make an application for your NINO. You can find further information on how to apply for your National Insurance number here on the Gov.UK website.
You do not need a National Insurance number to start work but you will need to get one once you have started a job. Your employer will give you a temporary number while you wait for your permanent number.