To register you’ll need:
Your reward from registering will come in the form of a Unique Taxpayer Reference Number (or UTR Number). This is a 10-digit number that HMRC allocates to those who need to file a tax return. Be careful not to leave it to the last minute as trying to get hold of HMRC during peak filing season is very difficult.
In the meantime, a good first step would be to keep your financial information handy, right from bank and investment statements, old invoices and expenses to P60 and P11D forms.