You will need to register with HMRC to tell them you need to submit a Self Assessment tax return. You must register by 5th October after the end of the tax year where you are required to file a tax return. For example, if you need to file a return for the year 2022/23, you will need to register by 5th October 2023.
Follow this link for registering “Step by step guide to setting up a personal tax account (bksaccounts.com)”