A person registers under GST in december 2018 as he thinks his turnover will cross 20 lakhs and he got the registration certificate and number, but still his turnover is less than 20 lakhs, now at what point he has to collect gst ,even the turnover is less than 20 lakhs he has to collect since he is registered or whether he can file a nill return upto his turnover is 20 lakhs and after 20 lakhs he can collect. If possible pls support with gst notification/law.