I assume it's not computer related coming from your previous comments, obviously you don't want to reveal what it exactly is which is fair enough but it does make it more difficult to advise.
This may help:
http://www.hmrc.gov.uk/selfemployed/
In regards to setting up a business and being self employed the following might help you
I can tell you exactly what to do from experience:
-What type of business will you be classed as? Sole Trader, Limited etc etc
-Will you be registered under your name or will you have a dedicated business name, this will also depend on the above
-Register as self-employed within 3 months of trading
-Register and setup for paying and filing tax online
-You will have to pay class 2 NIC's (national insurance contributions, £31.20 per 3 months) you recieve a letter asking for payment, can be done by a simple bank transfer although I'm sure you can set it up online, just haven't got round to it myself
-If your taxable profit is over £5,715 per year then you also have to pay Class 4 NIC's as well as Class 2, Class 4 is at 8% paid yearly with tax
-You also have to consider registering for VAT which is recommended for a reputable business and you have to if turnover is over £70k~ per year. This will again mean you have to supply VAT invoices to customers and keep VAT invoices from payments you have made. VAT is pretty simple to fill out online tbh, just make sure you keep records (this is done every 3 months) you will then either pay HMRC that VAT you have recieved on sales or if you have paid more on VAT (buying parts and other shiz) then they will give you the excess money
(Filing tax and VAT, as well as NIC's can all be done from one HMRC account it is all linked together)
You should also consider the following:
-Distance sellings regulations (unless a custom product or item which due to nature cannot be returned) Read 13. here
http://www.legislation.gov.uk/uksi/2000/2334/contents/made
-Warranty & testing
-Shipping & Insurance
-Recieving payments
There's probably more that can be added but this is the majority of what you need to know and take into consideration. It will take you more time than you think, getting your head around the above initially is very confusing but once around it, it will be fused in your head and will just seem stupidly easy.
Hope it helps, PM if you need.
