If you need to file a tax return for the year ended 5 April 2018, and you have done nothing yet, then the first thing you will need is a Unique Taxpayer Reference. This is a 10-digit number in a format like 12345 67890 and you need to apply for one. It is not the same as your National Insurance Number.
Any accountant can apply for a UTR for you, so see an accountant at once !