It may seem obvious, but if you want to write a romance novel, you should make sure that what you're writing really is romance and not some other genre. Romance books can include a wide variety of elements, but the one thing they have in common is the fact that the central story focuses on the relationship between the hero and the heroine and their path toward love. If the main story of your book isn't the love story, then you aren't writing a romance. Also, nearly all romance books have happy endings. If your hero and heroine don't end up together, then you'll have a hard time selling your book as a romance.
There are many stereotypes and cliches that you should try to avoid when writing your romance. Your hero can be strong, machismo and imperfect; plenty of women are attracted to "bad boys," and it's OK for your hero to incorporate some of those elements. But he shouldn't be so bad that your reader starts thinking your heroine is an idiot for being with him. He should have enough redeeming qualities that it makes sense for her to be with him, and your reader should hope for them to get together.
Another cliche is the dumb, helpless heroine, and this is another stereotype you should avoid. Your heroine can be foolish at times, and she can misunderstand the hero's signals or be confused about how she feels. But she should never be so stupid that your reader stops identifying with her. Her mistakes should make sense, and her motivations should be real enough that readers can imagine feeling the same way in her situation.
General fiction allows for wide variety in length, vocabulary and conventions. Romance publishers, however, are often very strict on what forms of romance they publish. Some publishers require a specific length for novels; others have rules regarding words that cannot be used in their novels or conventions that have to be followed. In addition, there are many subgenres of romance, and different publishers accept different subgenres. For example, Harlequin publishes paranormal romance in the imprint Nocturne Bites, historical romance in the imprint Harlequin Historical, and young-adult romance under Harlequin Teen. Medallion Press publishes paranormal/fantasy romance, contemporary romance, romantic suspense and historical romance. You should understand the standard conventions of romance subgenres and know what you're trying to write. You can incorporate multiple subgenres into one novel (for example, you could write a paranormal romance with suspense elements), but before you change conventions, make sure you know what publishers and readers usually expect in the genre.
It goes without saying that romance is a form of fiction, and so, in addition to following the conventions of romance genres, you should follow the rules of good fiction. Your story should have strong characters with realistic motivations, a setting that's interesting and detailed, and an interesting plot with a surprising twist that sets your story apart. Your characters should seem real, not stereotypical; the good guys should have flaws, and the bad guys should have redeeming qualities. Your plot should be driven by emotional conflict. External conflict (such as a killer or a hurricane) can be important to the story, but the main story should be the emotional conflict that ultimately brings the hero and heroine together.