The PAYE code for 2012/2013 before any deductions for benefits received or attachments to earnings is:-
This means that you can earn £8105 per tax year 06 April to 05 April before paying any tax. This is usually worked out on a monthly basis being £8105/12 = £675.41. So you can earn £675.41 per month before paying tax.
This does not apply to NIC please see Class 1 NIC Rates 2012/2013 for more information.