If you're employed by a company, you'll pay 12% of everything you earn over the £157 a week threshold on National Insurance. This will be taken out of your salary automatically, so you'll see it in the 'deductions' column of your payslip.
As an example, if you earn £250 a week, you'd pay nothing on the first £157 and 12% on the remaining £93. That would make your weekly National Insurance contribution £11.16.
If you make over £866 a week (£45,032 a year), you're considered a 'higher earner'. That means you will pay the basic rate of 12% on your earnings between £157 to £866 per week (being the Upper Earnings Limit) , and any earnings in excess of the Upper Earnings Limit will be subject to National Insurance at the upper rate of 2%.