Gstr 3b is apply which person opting compositon scheme

I AM APPLY FOR NEW REGISTRATION IN GST ON 30.06.2017, AND APPLY FOR COMPOSITION SCHEME ON 21.07.2017, IN THIS CASE GSTR 3B FILL KARNA HAI. (GST PORTAL LOGIN ME COMPOSITON WALO KE CASE  ME QUARTERLY RETURN DIHK RAHE HAI,  PER IS CASE M MONTHLY RETURN DHIK RAHE HAI) (GST CUSTOMER CARE WALO KA KEHNA HAI KE GSTR 3B FILE KARNA PAREGA. FOR FIRST 21 DAYS OF JULY BUT COMPOSITION SCHEME TO FULL FINANCIAL YEAR KE LIYE HOGA NA) MUJE KYA KARANA CHAHIYE.