Nil returns in gst

Dear All,

Before GST we have been regitered with Service Tax and rasing service Invoices without charging service tax, since our turnover was less than 10 Lakhs. Now we have been migrated from Service Tax to GST, please clarify whether to charge GST on all service invoices even the turnover will be less than 20 Lakhs (Treshold in GST). Also let me know how to file NIL returns in the GST portal if we have not charged GST on the Service Invoices.