Yes you need to be paying tax on your commissions. You will need to sort out a tax return if you are in employment. If you are not in employment already, then you should register as self-employed (if your earning merit it) so as to sort out stuff like your NI and that as well. If you’re earning more than £60,000 in a year from commission, you will need to VAT register as well. 
The best thing to do is fun up the HMRC and check with them, they can get a tax return sorted out for you or get you down as self-employed, etc.