I think this is what you need https://online.hmrc.gov.uk/registration and https://www.gov.uk/government/publications/self-assessment-and-national-insurance-contributions-register-if-youre-a-self-employed-sole-trader-cwf1
And as for Unique Taxpayer Reference (UTR), you would find that on any HMRC document eg. your previous Tax Returns papers, its a number around 10 digits.
And indeed if you can spend time to sort your own taxes, if you are a sole trader / self employed, thats best, in my opinion. It pays to know what you are paying the Gov!