Yes you do, another area to be aware of are the Distance Selling Regulations, which do apply to you even as a sole trader if you sell to consumers. You must comply with them, there are strict penalties if you dont. It does not apply if you ONLY sell to other businesses.
http://www.oft.gov.uk/Business/Legal/DSR/default.htm
HMRC are extremely helpful with tax matters and it's free advice, it's not complicated to be a sole trader and certainly nothing to worry about.
Ensure you keep proper records though to protect both yourself and your customers. At a minimum keep a list of all money that comes in and out, from and to whom, dates and references. Preferably though you'll use basic double entry accounting to simplify things if you get audited (it happens).
Brief yourself on self assessment tax returns too, as you'll need to complete one at the end of the tax year most likely. Be aware that your initial tax payment could be doubled, because you normally make two payments on account after your first year as a prepayment against next years tax liability. Good advice is to save 22% from everything you take from the business as personal income and put it in an ISA for example, that way your tax bill wont be a shock to the bank balance
http://www.hmrc.gov.uk/sa/selfemp.htm
One more thing, dont forget to have proper terms and conditions and if you collect user information, a privacy policy. Also, you may need to register under Data Protection. For this DONT use an agency as there are many scams where they try to charge a silly fee, ensure you are dealing with the proper government department if you need to register.
This lot shouldnt put you off, plan it properly and it's no problem, just dont put yourself in a compromising position by missing things out
Again, the above is only my personal opinion and does not constitute any legal or financial advice. It is a suggestion only.