When do i get taxed on a job?

You will get taxed in every paycheck. Welcome to the workforce! However if you make under a certain amount a year, I think @ 4,000$. You will get the money that was taken out back at the end of the year when you do your taxes. Have your parents look into that.

The employer will deduct tax and National insurance contributions (NIC) at source (at source means they take it off you before paying you - but they MUST provide you with a payslip showing the reason for any deductions by law). The employer calculates how much tax is due based on something called a tax code. The chances are that you are on an emergency tax code; if so you will see it on the payslip as 'BR' which means they will be charging a flat rate of 20% on all your pay. It is unlikely this is what you have to pay eventually, but it is what the employer has to impose until they are given a different tax code by HMRC.

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Did you sign a form called P46 when you started? - this is signed by you to show that 'this is my only or main job' and that you have not had any other income before this (earned income = mum and dad contributions do not count). The employer sends it to HMRC and the tax office should then send your employer a tax code - when they have done this, your employer will put something like 645L on the payslip, instead of BR. This may already have happened.

So long as they do this before the end of March 2011, any overpaid tax will be 'sorted out' in your payslip - ie, you will get a tax rebate if one is due. If they do already have the right tax code all the comments above are irrelevant (my description is of a common experience of people starting work however).

If your earnings are quite low (say below £71 a week) there will be no NIC due in that week. I very much doubt there would be tax either - you have to earn more than £6450 in the year to pay tax at all. That is well over £120 a week. Even then, tax is only a proportion (20%) of the amount you earn OVER £120.