1) Finish your manuscript. Butt In Chair. Sit and write until you type "The End." YES! You can do it!
2) Join a national organization like SCBWI (Society for Children's Book Writers & Illustrators), SFWA (Science Fiction Writers Association), RWA (Romance Writers Association), etc. - whatever is relevant to your book. These are incredible resources with incredible people. Be sure to go to regional events, conferences or national extravaganzas! The forums and online resources can help you get things you'll need like critique partners, query help, business advice and getting a clue.
3) Did I mention critique partners? Find some. It may take a bit to get the right fit, but these fellow writers are the critical eyes we need before we parade our stuff before the pros. They are also the ears, shoulders, and support structures necessary to make it through this process with our brains intact. It's my philosophy that I want 5 people not related to me by blood or friendship to tear a manuscript to shreds before an agent or editor ever sees it.
4) Did I mention getting a clue? Follow the pros online. No, I don't mean stalk them (which is bad behavior online or at conferences! Read my lips: No, no, no, no.) - but check out some of the helpful folks in the business who blog or share tips with writers-to-be everywhere. Whether agents like Nathan Bransford, Janet Reid, Kristin Nelson, Sara Crowe, Jennifer Jackson, or "Daphne Unfeasible", entire agencies like Dystel & Godrich, editors like Cheryl Klein and Andrew Karre, or anonymous tipsters like Moonrat, Editorial Anonymous, and the incredible Miss Snark; these folks take the time and energy to help you get it right. It pays to listen to them. Daily.
5) Find your community. Whether it's Verla Kay's Blueboards, Absolute Write, a LiveJournal or Blogger group, or the SCBWI Forums, there are great discussions happening that will give you a heads-up on the latest news in the industry and important tidbits you can get from those who are further down the line. It's amazing how much people are willing to share and help out one another. If you frequent these, remember to Pay It Forward, too.
6) Send stuff out. Where? Well, you've probably checked the forums and the bookstores to see where similar authors have succeeded: start there. Research agents and editors, check out blogs or interviews, read books from their client list and see if it's a good fit. Plug in your info at AgentQuery.com, QueryTracker.net or double-check for scams at the gold star Preditors & Editors database. Note requested submission guidelines and follow them. Pay attention to expected turn-around times so you don't freak out waiting. Once you write and polish your manuscript, write and polish your query letter, and write and polish your "elevator pitch" hook, the most important thing to do is lick that stamp or hit "Send." I can attest that no one will ever discover you if you don't do that. (And keep track of what has been sent where and when in a notebook or handy Excel or Google.doc file so nothing gets away from you.)
7) Keep writing. If only to keep sane while the clock is ticking and you check your mailbox over and over and over and over, it's also practical: writers write. Pinning all your hopes and dreams on one manuscript can tie you up for years (and I do not exaggerate here) so it's best to keep learning and expanding and dreaming on the page. Build up your kitty of cool Works-In-Progress so you have something else to offer in case someone asks, "So, do you have anything else?"
Does it look like a lot of homework? It is. I once naively thought that my job was merely to write good books (and that *is* my top priority) but in order to share any book with the world, we writers need help and we need to know how to get it.
And this is just the beginning...
NOTE: If I left anything out, please add it to the comments. The more info, the merrier!