You need to speak to
HM Revenue & Customs: Home Page and tell them your NI number and say you want to go self employed.
I'd chose a name and consider checking companieshouse.gov.uk to make sure one isn't currently running with the same name. Then I'd even consider (and we were discussing this the other day) getting the Ltd company of the same name and then keeping it dormant. So you'd be filing for that annually saying it's not traded and still just filling in your self assesment for being sef employed. But the day it gets a bit bigger than just you, you have the limited company ready to trade.
Speak to a couple of banks, setup 2 bank accounts, one for takings, then transfer about 25% of that figure each month to your other bank account which can pay your tax etc. Once you've offset some stuff against your takings you'll find you'll have left over cash in that account which I'd keep for rainy days or a holiday every now and again.
Grumpygrouter is a resident DJ, I mean accountant, and he will know best.
I'd get a website from mistore, and optimise it, and I'd get some reading done in the marketing forum which will give you plenty of marketing tips etc.