I did work as a science fiction editor for sixteen years before coming to Wiley, and I’d tell a would-be author one thing to begin with: fiction is very unlike non-fiction in how projects are sold. No first novels ever sell before they’re completed, so the first and most important thing is to finish writing the book and get it up to a professional level.
Once the book is finished – not just with “The End” in place, but actually as good as the authors can make it – they should start to research agents. (Don’t contact agents before you have a complete book ready to show them – if they get interested, you’ll just lose them as possibilities once they discover that the book isn’t done.)
Some good links to read about agents and the field in general:
Those links are a good place to start researching agents; another is through the works of similar authors. Aspiring SF authors should do a little market research, by seeing what else is out there that’s similar to their books. Make a list of authors that write books in the same subgenre (alternate history, space opera, military SF, whatever) and also try to find out their agents – authors often thank their agents in the acknowledgements in their books, and even more often list their agents as contacts on their websites.
Once you have a book that’s good enough to publish and a list of agents that have represented successful, published books in that area, it’s time to match the two up: go to the agents’ websites, and look for their submission guidelines. Then follow those guidelines precisely – fiction agents can get grumpy about non-standard submissions because they see huge waves of submission material. So if one agent asks for a synopsis, and another asks for the first ten pages and a synopsis, and a third asks for the first chapter but no synopsis…do it the way they ask, because that shows that you’re a professional.
Again, there are literally thousands of people writing novels for every published writer. The bad news there is that good work has to stand out from the flood. The good news is that salable work does stand out strongly from the flood; if you can tell a good story with interesting characters, to the level that people who don’t already know you want to pay money for it, you will get published eventually.
But I’d also say that, for most writers, the first novel they complete is not the first one they sell – and the ones that succeed are the ones who want to tell stories, not just have one idea that they’d like to see become a novel. It’s not unheard to publish the first novel you write – particularly, like in this case, when you’ve been a professional writer of other things – but fiction-writing is different from non-fiction in a lot of tricky little ways, and a novel is a long story with a lot of ways to go wrong in the middle. If you do really want to write fiction, don’t get discouraged – there will always be more stories, and each story teaches you something about your writing.