If you employ someone - even if it's only yourself - you'll usually need to register as an employer with HM Revenue & Customs (HMRC). Most new employers can register online, but some will need to register by email, by telephone, or with an HMRC office.
The following link explains who needs to register as an employer, what information you'll need with you when you register, and how to register. It also explains who will need to register by email, telephone or with an office.
Read more on registering and getting started with PAYE.