When you become self-employed, you will need to complete a self assessment tax return every year and pay your own National Insurance Contributions
To do this, you need to register for self assessment with HMRC. You can find step-by-step instructions on how to do this on the gov.uk website: Register for Self Assessment
It's easy to register online, and you only need to provide a few personal details, like your National Insurance Number & some information about the work you're doing.