If you're making more than the personal allowance a year, (around £5k) you need to pay tax. If you're just a kid making money from garage sales, washing cars etc, chances are the taxman couldn't give a flying feck about you. However if you're making £xxxx a month online its time to do something about it. If you're banking CJ cheques I'm amazed the bank haven't said anything. They generally take a look at what you use your account for - if its a current account like mine they'll say strictly no business use. I'd do what the other suggested. Go see an accountant or financial adviser. Register a business and open a business bank account. Pay an accountant a few hundred a year and he'll keep on top of everything for you. Just give him copies of invoices, reciepts etc.
I'm in the uk and I work fulltime. I also rent out one house and I have a limited company. Get a good accountant- you will of course have to pay some tax, but you can offset many things against the income. I pay £280 for my personal accountant and £400 for my limited company. If you earn money the inland revenue will find out about it one way or another and so it is best to be straight and pay your taxes, being chased by them is not a nice experience I believe!