Your auto enrolment duties start as soon as you employ someone for the first time, even if it’s only one person. You have to set up a workplace pension and assess them for auto enrolment into it.
Not all your employees will be eligible for auto enrolment. But you still need to assess them as part of your auto enrolment duties, and send them an assessment notice telling them what their rights and options are.
The Pensions Regulator has a handy online checklist to help you work out what your duties are.