I've written a few books over the past 10 years or so, and in that time I've been learning about publishing, about what writers can do to help themselves out, and about the basic, professional standards in the industry. Now, I lived, studied and worked in England for the majority of this time, so this advice is UK-specific, but I'll mark US-equivalents where necessarily.
This all applies to novel-length fiction, by the way.
You've got to write a book, first. Surprisingly, this isn't something everyone knows or does. Literary agents (of which more below) and publishers do get approached by prospective authors or with a 'pre-query' (which is not a thing) but all that does is show you haven't understood how the system works. Non-fiction is different, but that's not my field. Fiction is, and in fiction, you gotta write. So, write your book, and revise it, and make it as good as you can. Only then move on.
Written it? Great! Congratulations. You're already doing better than most. Starting a book isn't the same as writing a book. Awesome.
You can approach a publisher directly, but this is almost always going to lead to a form rejection. In today's world, most editors are simply too busy to take on un-solicited manuscripts (mss.) Un-solicited in this case means 'not requested,' and any mss. sent in like this go on the slushpile, or the stack of books waiting to be read and / or responded to. I'd avoid publisher submissions for now, and instead aim for a literary agent.
And here's the first link - you need to buy The Writers' and Artists' Yearbook. Published annually, it lists all the agents, publishers, and everything else you'll need, along with explaining how to write to them, complete with samples, advice and pretty much anything you could need to know. (The US equivalent would be The Writer's Market. Both this and the Yearbook are available in most bookstores)
The 2013 edition will be out very soon. Read through it, see what agents are accepting your genre, and do what the books say, exactly as they say. Professional writing needs professional approaches. Literary agents will be able to submit your ms. to publishers - the normal rules don't apply here, if you like, and they have a more direct line to the editors. Most of them are friends, or colleagues at least. They add professionalism and gravitas to your submission, and you pay them with a cut (15% is normal) of any money you make. Go on their websites and follow their guidelines. You have been warned.
Here's my second linky! Getting an agent itself is hard, with competition from other writers far outstripping the amount agents can handle. How can you give yourself a boost, keep up to date, learn the best practice and get great advice from those in the know? Get involved in the writers community on the Absolute Write Forums. It's invaluable, I promise you. Your query and submission will be better, you'll be more informed, and you'll be happier for it all.
Once you're submitting, I recommend Query Tracker as the easiest way to keep up with who else is walking the same path as you, agent response times, and agent moves.It's free and pretty easy to use. Response times tend to hang around the three month mark, so try not to go crazy, and keep busy. Start your next book, for one thing.
Through all of this, keeping up with the latest book news is a good idea, since you want this to be your profession. The Guardian Books site, for all its flaws, is one of the best I've found. See what's happening to authors you know, in the industry at large, and learn about the newest trends.
And finally - what do good writers do? Read! Read widely, in your own field and outside of it. Read the latest prize winners and best sellers, and then read whatever else you want to. Reading is learning, I guess. Learning craft and style and words you can steal later.
It's hard, but the part where you write and read should be fun. Remember that most of all. If you're not enjoying it, what's wrong? Are you isolated, or have you misunderstood what to expect? Turning a hobby into a living is never easy, and if you don't have a love of writing for writing's sake, that could be a indicator of trouble to come. Stay positive!
And that, for now, is it. Do you have any other links or tips to share? Let me know in the comments below - I'd love to keep learning more myself.