To be honest, any business runs roughly the same way, I run a plastering business, MD Plastering Services (Anyone need a reskim :) ), it's only the product that's different at the end of the day, you offer software, I offer, well, plastering.
keep a book of your transactions, or to use a bank account that records it
What Grey said
Do you have to register as a business or have a tax ID or anything official like that? Or register your company name?
Visit your local tax office, because my business is in construction, I had to apply as a CIS (Construction Industry Sceme) card holder, also, I had to declare if I was a sub-contractor or a contractor, I'm the latter, which means I can hire people, though, I have to pay their taxes for them.
Not sure about IT, but I imagine it's along the same lines... you either work for someone programming as a sub-contractor and they pay your taxes, or you work for yourself, which means you have to declare your own... It's not that hard, once you visit the tax office, they send you the required information
EDIT: I also get £0.40 a mile back in tax, I roughly do 400 mile a week, wahey! :)
EDIT: And don't forget, you'll be eligible for 'new business' grants too of the goverment (If you qualify)
What about working from home, if your home becomes your office, isn't there some laws about that?
Same as Grey, you can even claim the VAT back from your bog roll if you want! :)
And websites
Don't just rely on the internet, get your product into as many publications as you can, PC Format review independent developer games, contact them, you never know. Even go so far as hiring yourself freelance in your local paper(s), you never know!
Dabz
PS There's also VAT, as in, being VAT registered, I'm not at the moment, though, I do remember Grey saying something about he is.
Quick run-down on being VAT registered Grey? ;)