If you're self employed in the UK you'll need to register for tax self assessment with HMRC. You'll be able to check whether you have to make a Payment on Account - a partial tax payment installment during the tax year - or pay in arrears according to your income level. Deadlines can change over time. For the 2019/20 tax year, you'd need to register for self assessment by 5 October 2020, and pay your taxes in full by 31 January 2021.