Lot's of good advice on this thread already. I'd just like to add a list of the most basic steps you should take.
Firstly, make a business plan and decide what exactly you're offering, why people should buy it and who those people would be. Then it is easier to figure out the promotional side, as well as the finances.
Secondly, look up the self-employment tax rules on the HRMC site. You need to register for self-assessment.
Thirdly, check what kind of insurance would be applicable to you. There are some good guides online about this, e.g. this guide on public liability insurance. It's easy to make mistakes when you're just finding your feet, so make sure you protect yourself.
Fourthly, read lots and lots of materials online!
Best of luck!