My 17y old niece is working part time,do you have to earn a certain amount(per week) before you start paying tax or do you pay from the first euro earned?
She is probably on Emergency Tax, and hasn't provided a PPS number.
Get her to give her employer her certificate of tax credits (or P45 or P60 - although this is probably her first job). She will then be taxed on a cumulative basis and receive a tax refund.
[broken link removed] (note the links at the top of the page) contains some useful information including some on emergency tax in case that's of any use.