I will reply to everybody - eventually. But in the meantime I thought it might help for me to put up here three really good resources for anybody who wants to write professionally.
Once I've recommended these, there's not much more advice that I can offer to help you get published (actually, that's not true - I have LOADS more advice, I'm just going to save it for another blog post at some point later).
Anyway, here are three websites you'll find useful.
First, the big daddy. If it weren't rude to respond to people with emails of a single sentence, I would just write: "Get a copy of the Writers' & Artists' Yearbook." As it is, I usually have to pad out that reply with some "ooh" and "aah" and "hello" and "how's your knee?" etc.
Here's the link to their site: http://www.writersandartists.co.uk/
Next up, a website that's evolved a bit since I started recommending it, so check it out and see what you think. This one is a little more US-centric, but still packed with useful information and chit-chat: http://www.authoradvance.com/
Finally, a new site has popped up that looks incredibly useful if you're interested in writing for children. More than that, it looks essential. It's very clearly laid out and well written, and it even has an easy-to-read list of all the main UK publishers of children's books. Have a little look:
Now, you shouldn't be sending your stories straight to these publishers, but you should get yourself familiar with what they're publishing and what they're interested in. Browse around the rest of the site to see which books are big, which are well-reviewed. Hey, you might even want to read some of these books.
Then use the first two links to find an agent.
Oh, and how's your knee?