Firstly, you need to work out which tax year your start date falls into. The tax year runs from 6 April to 5 April, so your start date falls into the tax year ended 5 April 2012. You must therefore register by the following 5 October, i.e. 5 October 2012. As you are registering as self employed, the form you need to complete is HM Revenue & Customs form CWF1.
You will also need to pay Class 2 National Insurance which is only £2.50 per week for 2011/12 so most people choose to pay for these contributions via Direct Debit. You will need to complete HM Revenue & Customs form CA5601 if you would like to pay via this method.
Although you have some time before you need to submit form CWF1, avoid leaving it too long. 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.