Online ad-hoc work; how to pay NI?

I'm a UK resident and have been trying out some ad-hoc online work for a US company that pays in dollars. So far, I've only earned $160 but I can foresee earning the equivalent of £25,000 to £30,000 per annum. I've only ever been a salaried employee in the past, so this is new to me. I'm pretty sure that I need to pay NI if I earn over £12,000-ish but how? If my circumstances change and I earn much less than anticipated (i.e. less than £12k), will I still need to do something formal to declare that non-taxable income? Anything else I need to know?

Hi,

Anything over £1000 would need to be reported as your personal allowances would already be used against your PAYE income. This will then determine the NI that is due.

Thank you.