Employers with 20 or more employees should have been reporting STP from 1st July 2018.
Legislation has passed for smaller employers of 19 people or less to start reporting between 1st July 2019 and 30th Septemner 2019. (earlier reporting is an option) Low cost options are available for micro employers (1-4) employees who don't currently use payroll software.
Not long to go now to ensure you're ready to report!