Most people have to pay National Insurance as it counts towards benefits you may be able to claim in the future such as incapacity benefit and your State Earnings Related Pension Scheme (SERPS).
Most UK citizens have a National Insurance Number sent to them automatically at the age of 16. It has 2 letters followed by 6 numbers and then one more letter. It is important to keep a written record of this number as it is yours for life.
There are different levels of contribution and the amount you pay will depend on how much earn. You can find out more - plus how to apply for a National Insurance Number if you haven't got one from the Department of Work and Pensions.