My DDs being nearly grown up now (sniff) I decided it was time last year to do what I've always said I wanted to and Write. I started off by sending a transcript of my blog (the In-thing, someone just sold hers for 70K) to a publisher. Amazingly, it nearly made it... the commissioning editor loved it but it tripped up at finance board level over that usual obstacle 'not commercial enough'. In the meantime (in writing, everything has a very sloooooow timescale and you have to wait months) I started writing short stories for women's magazines and have sold ten to date - though that sounds better than it is, I have also had a LOT of rejections, I aim to get one out of four accepted so it's hardly ever going to bring me in loads of cash - just some useful pocket money and the thrill of feeling like a Real writer.
I also write little things for newspapers (have had a few short things in the Guardian - Family - you know the Snapshots/We Like to Eat/Playlist section?) - I heartily recommend this to have a go at, they pay £75 for a couple of paragraphs if you can hit the right note. Ditto Reader's Digest - £100 if you can come up with something for an end of article filler.
My top two tips would be a) Persistence - expect lots of rejections, start work on the next piece the moment you send out anything completed and B) really do study the publication first to see what they are looking for. I wasted lots of time and effort NOT following that advice - eg I sent off several stories to Fiction Feast which were written in the first person, only to see from their guidelines that 'we hardly every publish stories written in the first person so don't waste your time' - groan. Many mags will send you Writer's Guidelines if you send them an SAE.
OK but the OP was about writing a book, so back full circle: my blog is with another publisher who is making me wait an agonisingly long time over it - she phoned the other day to say she would make a decision in SEPTEMBER (she has had it since January!) so that's on hold. And yes I'm also having a go at Mills and Boon, my aim is to send them the first few chapters and see if they tell me 'yes you're on the right lines' or 'don't waste your time.' I wouldn't advise doing this with most other fiction as they'd expect you to have a finished book to offer, but the Mills ad Boon site appears to invite a speculative sample which will save a whole lot of work if I turn out to have no talent at it.
Sorry this is long and hope it doesn't sound 'braggy' - honestly I have had oodles more rejections than success - and as for getting a literary agent (essential if you want to submit to the larger publishers) well, I have tried five and all have turned me down flat with little or no encouragement.