Hi Indi,
If you do not yet have any P45, for now you should tick Option B for the employment you think maybe the main employment, and option C for any subsequent employments, the standard personal allowance this tax year is 12570 , this is shown on your payslip under the tax code 1257L, what this means is that every tax month - Apr 24-Mar 25 you can earn upto £1047.50 before any tax will be deducted. And any unused amount is carried forward to the follwing month.
So if you earned say 547.50 in Apr 24 then you will have 500 spare tax free earning that will be rolled over to May 24 pay, so that month you will then be able to earn £1547.50 before any tax will be deducted, this carries on like this for the full tax year.
If you have 2 employments usually the main employer has the tax code 1257L and your second operates BR (20% tax) on all earnings there.
You can, if you know what you income will be at both sources and once they have started, ask HMRC to split your allowances between both sources.
Income Tax: general enquiries.
Thank you.