If your annual revenue, based on calendar year is nearing or had exceeded SGD1,000,000 , you need to register your business to be a GST registered with IRAS.
As such, your bill to each customer must include the component of GST output tax at the prevailing rate.
All local purchases with local vendors that are also GST registered will also be recorded with the GST input tax.
At the end of the quarter, close your input tax and output tax account to a GST control account ( Debit or credit balance)
If receivable, you will expect payment from IRAS to your organisation
If payable, you will need to issue a payment immediately IRAS when it is due
Click here this link to IRAS GST website for more information