How should one introduce automation testing to a software which is already in production?

How should one introduce automation testing for a software (Android & iOS) which is already in production, just being maintained through release cycles (fixes and new features)? Such software was only undergoing user acceptance testing which was being done manually. Since the system is already built, which would be the most efficient approach to introduce automation testing? From where should one start? Which types of testing would one ideally start with? Thanks