Hi tonynsabine Sabine,
As this will be your first employment, you should advise your employer that you will need to complete a 'starter checklist', as you do not have a P45.
Starter checklist for PAYE
You will need to obtain a National Insurance number, which you can do here:
Apply for a National Insurance number
This will allow a cumulative tax code of 1257L to be applied against your employment income, which will backdate your personal allowances to the start of the tax year, so that you should have paid the correct amount of tax by the end of the tax year.
Thank you.