If you haven't registered as self-employed already you must do this. You must register within 2 months (from memory) as soon as you start trading. You are liable to a fine if you do not register after this time.
You will need to read the forms about whether you should pay National Insurance Contributions. In addition whether you need to pay any tax on your incomes. Each situation is different, get in contact with HMRC if you have any doubts.
