This is a paraphrase of something I heard Neil Gaiman say when asked for advice for new writers.
Step 1: Write something. So many people want to be writers, but never actually write anything, so they never know if they're any good. Maybe all they want to do is think about it, and that's fine. But if you want to publish a novel, you need to write one first.
Step 2: Finish the darn thing. Write it all the way to the end. Then stop fiddling with it. If you spend too much time messing with your work, you'll never get to...
Step 3: Submit your manuscript for publication. Find a publisher that publishes the sort of fiction you've written. Check the current edition of The Writers' Market for a nice list. Make your manuscript meet their requirements, and send it to them.
Step 4: When you get rejected, send your manuscript to another publisher. Repeat. Repeat. Keep repeating until you find your publisher. If your work is any good at all, and maybe even if it's junk, someone somewhere will publish it. Someone out there is clever enough, perceptive enough, brave enough, stupid enough, drunk enough, or angry enough to publish your work. Maybe the guy has just learned he's going to be fired, and your manuscript is his revenge on the company. Who cares? You're getting published, right? You job at this point is to find that person. This may take some time.
Step 5: Do it all over again, but different.
Caveat: I have never done this myself. :-)