If you're self-employed you pay 'Class 2' NICs at a flat rate weekly amount of �2.20 (there are also special Class 2 rates for share fishermen and for volunteer development workers).
You also pay 'Class 4' NICs as a percentage of your taxable profits - you pay eight per cent on annual taxable profits between �5,225 and �34,840 and one per cent on any taxable profit over that amount.
If your earnings in the 2007-2008 tax year are expected to be less than �4,635 then you may be entitled to the Small Earnings Exception (SEE), meaning you don't have to pay any Class 2 NICs.
http://www.direct.gov.uk/en/MoneyTaxAndBenefit s/Taxes/BeginnersGuideToTax/DG_4015904