You could be taxed on the first, say, �200 you earn in a new job. However if your gross earnings in a tax year are less than �4895 then you can claim back any tax that you have paid.
Gef is correct. I should have pointed out that this is your yearly personal tax allowance and that this figure would be different if other allowances were applicable to you or if you had other tax liabilities.
It depends how much you earn and how far we are through the tax year as to whether or not any tax is immediately payable. This may be paid back through your wages or reclaimed after the tax year.