Firstly, you need to work out which tax year your start date falls into. The tax year runs from 6th April to 5th April, so your start date falls into the tax year ended 5th April 2018. You must therefore register by the following 5th October, i.e. 5th October 2018. As you are registering as self employed, the form you need to complete is HM Revenue & Customs (HMRC) form CWF1 or you can register with HMRC online at www.hmrc.gov.uk.
Although you have some time before you need to register, avoid leaving it too long as you may face penalties for late registration.
Please note, this is merely with regards to self assessment, and does not include VAT or employer matters for example.
Your local TaxAssist Accountant would be happy to help you meet your responsibilities with HM Revenue & Customs, and can also help you to set up your records or put together a cashflow forecast etc.