You need to call your nearest tax office and ask for what you need. Used to be a form, but it's mostly online now, so they can tell you what you need to do. You just fill it in once a year when Adam Hart-Davis tells you on the telly. If you are going to be earning anything other than pockey money, it's a good idea to be safe. You don't want the taxman finding you owe thousands in backtax.
On the other hand, remember you can offset your costs like tools, sundries, lunch when away from your normal place of work, and petrol to travel to clients etc. I am sure you wouldn't claim anything which wasn't 100% kosher, but you are probably going to need a new laptop, and a decent phone to handle your new business - maybe even a camera to take pictures for your website, and a sat nav would be a handy business asset. Anything like that would come off your taxable income, so keep your receipts.
If you are employed as well you will need to complete the employed and self-employed sections, so keep your payslips / P60 from your employer. If you keep all your payslips, receipts, invoices and bank statements in one place, it will take you about an hour a year. (I have a different system which means I can do it in almost a week).
Also, just in case you do over the VAT limit (I think currently around 70k) then you really need to be clean on your returns. The taxman can fine you, but the excise guys can put you behind bars!