You should apply for registration at the earliest on the GST common portal and obtain application reference number (ARN). You need not disrupt your business and may continue to make supplies on invoices without GSTIN. The application for registration must be made within 30 days of the turnover crossing Rs.20 lakhs or attracting any of the conditions mentioned in section 24 of the CGST Act, 2017. After registration, you can issue revised invoices as permitted under section 31(3)(a) of the said Act. These supplies should be shown in the return and taxes paid on them.