To get your Unique Taxpayer Reference (UTR) number and become self-employed, you can register online via the following link:
https://online.hmrc.gov.uk/shortforms/form/CWF1ST?dept-name=CWF1&sub-dept-name=&location=40&origin=http://www.hmrc.gov.uk
Fill out the online form and make sure to do the following:
1. Register for Self-Employment Only
2. No not previously registered for Self Employment
3. Complete your business details as in personal details
4. Tick you are a subcontractor under CIS
It will take about 4 weeks to arrive in the post. Once you have it, you will need to activate it either online or over the phone (details and a password will be provided on your letter). Please remember you will have a limited period of time to activate your UTR before your password expires.