Integrated Airman Certification and Rating Application (IACRA)
Navigate to: https://iacra.faa.gov/IACRA/Default.aspx
You will need to register for an account. The process is straightforward and you will receive your FAA Tracking Number (FTN) once you have completed registration.
You will need the FTN when it comes time to test and to submit your application for your drone pilot certificate.