The moment you employ someone, auto-enrolment duties begin — there is no grace period to opt out of. You have to assess the worker, put eligible staff into a qualifying pension scheme, pay the employer contribution, and write to them explaining their rights.
The part most first-time employers miss is the paperwork trail: The Pensions Regulator expects a declaration of compliance, and the deadline for it runs from your first employee's start date, not from when you get around to it.