It's important to let HMRC know that you're self-employed as soon as possible - even if you already fill in a tax return each year. If you don't tell them as soon as you begin self employment you may have to pay a penalty.
Once you're registered as self-employed, you'll receive a Self Assessment tax return to complete each year so that you can provide details of your earnings and any other income you get during the tax year (6 April to 5 April). This information is used to work out how much Income Tax you have to pay.