As soon as you register for self-assessment or set up a limited company, you will automatically be issued a UTR number. If you can’t find it, you can also call HMRC on 0300 200 3310 to ask about your UTR number. You’ll need to have some information on hand for security questions, such as your personal details and your National Insurance number.
If you want to submit your Self Assessment tax return online, you’ll have to set up a Government Gateway account. Just follow the instructions in the letter containing your UTR.