If you are self-employed you will need to complete a self-assessment form for HMRC.  Click the link.

http://www.hmrc.gov.uk/forms/sa100.pdf

Here you will tell HMRC what money you have made and what you have spent on the business and work out how much tax you owe. It is entirely up to you if you have a bookkeeper or accountant keep your books for you or keep them yourself. There are bookkeeping apps available such as Quick Books or you could keep a spreadsheet with your sales income and expenses.  Self-Assessments have to be completed and filed by 31 January the following year so for tax year 6 April 2020 to 5 April 2021 you have to complete your tax return by 31 January 2022.