I presume you are in England then, well you have to fill in a form with revenue and customs and let them know you are earning a wage, you can earn money for upto x amount of months, I think it is three month before you must register as a sole trader, limited company etc. You can do it online, over the phone or by mail.
here is the form
http://www.hmrc.gov.uk/forms/cwf1.pdfI would register as a sole trader, or you will get stiffed on taxes if you go for limited before your over the threshold for it.
You might also want to send away for a national insurance exemption, if your earning for the tax year is under 4k or something.
I like many I think started in another job, and worked part time while I built up my round, I did that as much as I could then went full time. But even then I wasn't making allot of money, not what I wanted to take as a wage each month, and had to do a bit of round building to get enough money to make a living I was happy with. My point is you might not get there straight away, but work hard and you will get there.