Not sure what advice you are expecting from this site other than own up and contact HMRC. You will have to pay the back tax and some panalties for late payment, but I don't believe its worse than that.
In terms of the way the system works, its not that difficult. In round numbers, the first �5k of earnings has no tax due, the next 32k is taxed at 10% and after that its 22% (until one reaches far bigger numbers than your �12k, when higher rate kicks in). As self-employed, you'd also have to pay NI which is about 10% flat. Armed with that info you can work out roughly what you would owe.