The P30 is normally based on the amount of tax and Prsi that is deducted from your salary (you being the only employee). So if you deducted €500 tax and €100 Prsi in your salary, you would declare these on the form and file it with the cheque for the liability (if filing online, you can provide bank or laser card details for payment).
Bear in mind that the P30 isn't actually a declaration of a liability. It's a payslip. So if for any reason, you wanted to hold on to (e.g.) this month's tax/prsi and bring it up to date later in the year, you can. You use the P35 to declare the year's liability and pay any outstanding balance due.