Which Login to Use
First-time applicants have no login yet. On gst.gov.in, use Services → Registration → New Registration, fill Part A and get a TRN. Then come back to the same page, choose Temporary Reference Number (TRN) and log in with it to finish Part B; the TRN is valid for 15 days.
After approval, the GST portal emails your GSTIN and a temporary password. Use “First time login” on the login page to set your own username and password; from then on you log in with them to file returns and download the certificate.