The operation of PAYE means that the tax-free allowance is allocated monthly, but in the end tax is calculated on an annual basis.
If you haven't worked since the start of the UK tax year in April and your employer is aware of that, which they should be via the "new starter" checklist, then you should get the entire tax-free allowance for the year to date in your first payslip. You would then get the rest of it in monthly chunks through out the rest of the year.
So if your first payslip comes at the end of July, then you'll get 4 months' worth of allowance, i.e. £4,166 (1/3rd of the current annual allowance of £12,500). As your salary for the month would be less than that, you won't pay any tax this month. Next month, August, you'll pay a bit less tax than usual as there'll still be some excess allowance left. In September and onwards your take-home pay should stabilise to the standard rate.
National Insurance is always charged by pay period so you will be charged the standard amount immediately. Likewise your pension contributions will immediately start out at the standard rate. So in the first month you'll receive your gross pay, minus NI and pension. (Assuming there aren't any other deductions, but I can't think of anything likely.)
