Check what's on the shelves at bookstores and read book reviews. A subscription to "Publishers Weekly" can be useful as well. No one can predict what a publisher will or won't buy. And as far as the story goes, what constitutes a salable manuscript is subjective. But keeping an eye on the market, what the public is reading and what publishers are publishing, can give you a general idea about the type of story you should write and spark your imagination.
Put quality time into writing your manuscript, whether it be an hour a day or five hours a day. Write the first draft to capture the essence of your story, then do a second draft to correct problems with the story. These problems can be anything from key plot point issues to point-of-view issues. You might encounter problems with setup or scenes that aren't believable. You may find that your characters are too flat or your dialogue is too stiff. These are the problems you need to correct in your second draft, even if it means cutting away writing you feel is good. If it doesn't move the story forward, if it isn't crucial to character development, then remove it. Your final reading and edits should focus on mechanical errors: typos, spelling errors and problems with confusing sentence structure. The manuscript you deliver needs to look professional. A sloppy manuscript can cancel out a good story.
Get an agent. If you want to earn royalties on your book, your book needs to be published by a publisher that will promote and distribute your book widely. While it's possible to get a book published without an agent, having one can get you access to big publishers who prefer to work only with writers who have agents. Consider using an online service such as Agent Query, which offers a database of legitimate literary agents you can search free of charge. The database returns agency contact information based on your search criteria. Once you've found an agency you feel may be a good fit for you, compose a query letter and pitch your novel to them. Make the letter short. Introduce yourself, let them know you're a first-time writer seeking representation for a novel, give a brief synopsis of the novel, and thank the agency for considering you. Agent Query also offers many helpful articles and tips about agents and publishing.
Buy a copy of "Writer's Market" or subscribe to the Writer's Market online database. This database of publishers features complete contact information, information about how to submit your manuscript, what your royalties will be, and whether or not the publisher will work with unagented writers and first-time writers. You can submit your manuscript to any of these markets without an agent if the guidelines indicate it's acceptable to do so. This gives you the chance to find a royalty-paying publisher even before you find an agent.
Be persistent. If you believe your novel is worthy of publication, keep submitting, even if you are rejected several times. Rejection is a reality of the writing business, but you lose only when you don't try. Dust off any rejection you get and find a different publisher. If you get constructive notes from an editor, take them into consideration before submitting again. Those suggestions may be what get you published with a royalty-paying publisher.