You will need to register for Self Assessment if you are self-employed or have received additional income, such as through a buy-to-let property, savings, or if your (or your partner’s) income is above £50,000 and you claim child benefit. You must also register if you are operating a company in a Partnership.
If you did not submit a return last year you must apply by 5 October 2018, and you will need your national insurance number, address details, and residency status to hand in order to complete the registration process. Registration is encouraged online, however, should you be unable to do this you can print out the form, complete it, and send it to HMRC in the post. Following you submitting your application you will be sent a 10 digit UTR (Unique Taxpayer Reference) within 10 days. Keep this number safe as you will need it to submit your tax return in future years.