Not stupid at all mate, every business in the world started somewhere.
Get a website going first, this can be sitting around waiting for new customers whilst you get started. There's no point quitting a job on a Friday and sat waiting for a phone call on the Monday.
Knock up some leaflets to start off with, or considering a small ad in a local maagazine or newspaper.
You could start off by doing it at weekends, and if it takes off then considering doing it full time.
Image counts for a lot. Get some decent clothing, good workwear that's clean and tidy, with proper work boots. Nothing's worse than someone turning up to do some work at your house wearing tracksuit bottoms, trainers and a battered football shirt. I wear a green polo shirt, black combat-type/work pants and steelie boots. Counts for a lot when I turn up and I look the part.
Buy a seperate mobile for your business. Just a pay as you go. Nothing worse than being sat in the pub and your own phone goes and it's a customer ringing up for something. At the weekend the work mobile gets put in away and I don't check the messages until the Monday.
I do all my own accounting, and have my own web-based invoicing software. I'm considering licensing this out for people having to invoice more than say 20 invoices a week. I might need a guinea pig to see how it works, so give us a shout if you're thinking of going ahead.
Just be professional. I get so much work from neighbours of my customers or people passing by. I'm there in my work clothes, my van's clean and I do a good job. It's so simple, but it counts for a lot. Christmas cards to customers etc.
On the legal side, you don't really need to do much. All you need to do is let HMRC know that you're going self-employed, and you want (ha ha) to do a tax return at the end of the tax year, and they'll put you on the system. Doing a tax return is a piece of piss, I buy a £15 spreadsheet from diyaccounting.co.uk each year and I can do it in a few days.