I was employed by a company on a self-employed basis before.
You need to contact the Inland Revenue- just give them a call and they will register your self-employed status. The good thing about being self employed is that you don't have to pay tax as you go along- so you can stash the tax in a savings account and earn interest on it until you have to pay it.
You will also need to keep some form of accounts. This is most likely very simple in your case, just keep a record of all money coming in. Being self-employed you can deduct business expenses from your income, and you only get taxed on the outstanding balance (e.g. transport)- it might be worth your time setting up a simple account system to record 'business expenses' so you can deduct them before paying tax. You can even deduct some money for 'rent' if you work from home at all-- as well as telephone bills, etc.
You might want to get an accountant to do your end of year tax return for you- once you are registered with the Inland Revenue they will send it to you automatically. It is not necessary to have an accountant but if you don't like doing the paperwork they should do it for you fairly cheaply since there is not much work involved. I've filled my own self-assessment forms and they are pretty self-explanatory.
Don't forget you need to take care of your own National Insurance contributions as well- HMRC will tell you about it if you ask them. The minimum is about £2.50 a week- but you should top it up (at 6% i think but not entirely sure) if you ever intend to claim a pension or other state benefits.
Another thing to consider: VAT. If your 'business' turns over more than £67k a year you will be required to register for VAT. This is great for you as you can claim back all the VAT on your purchases. Extra paperwork- but worth it
.
I know it seems a bit daunting to begin with but in time you will get to grips with it. In the first instance the most important thing is to make sure you put aside any monies in a separate account for your tax each month. This is not an exact figure but as a rough guide i would put aside 22% of the first £34k... check the tax band for higher income rate i'm not sure of the exact amount? In reality, there is a personal allowance of around £6k a year before you pay tax at all, but if you follow the above, you will always have more than enough in your tax account- nothing worse than having a surprise tax bill sprung upon you!
There is lots of useful info on the government website:
HM Revenue & Customs: Self Employed
Register as self employed here (must be done within 3 months or you will get fined!):
HM Revenue & Customs: