Lifehacker has a smart post about what it's really like when you go work for yourself. Some of the details are very USA-specific – naturally, since Lifehacker is an American site – but the principles are the same here in the UK:
Often, people want to freelance or start their own business because they're lured by the freedom of working from home. If that's what you care most about, you're probably better off trying to convince your boss to let you telecommute and learning about the downsides of working from home rather than leaving your employer to work for yourself.
List articles – 5 things to eat, 10 things to sell or whatever – are usually quite lazy pieces of writing that are also pointlessly empty. And I'll give you seven reasons why. But this one is simple and straight and practical: I'm not sure I've ever read it all put so well as Lifehacker does.
I've been freelance since the mid-1990s but I also had an enormous crutch of a regular client for a dozen years so I felt I eased into this life. Can't imagine going back now, but I can imagine doing this freelancing an awful lot better: when you've read that article, follow its many links out to further advice. It's a smart collection.