If you want to write romance, you should read romance. This will help you both to learn the rules of the genre and see what others are doing so that you can avoid repeating someone else's idea. As you read, pay attention to similarities between books to determine what qualities are essential for a romance novel. Romances, for example, revolve around a growing relationship between two characters and are generally expected to have some sort of happy (or at least hopeful) ending. Learn as many of the rules of the genre as you can before you write your novel.
A hook is the characteristic of any story that grabs a reader's attention and "hooks" her to the book. Hooks are used to attract agents and publishers as well as readers. What makes your book stand out from other romance novels? What makes your story interesting to read? A hook can be anything from a dramatic action scene to an internal revelation about a character's personality, past or emotions. As a general rule, your hook should occur within the first one or two pages of your romance novel. Perhaps your female lead is already obsessed with a male character or she finds herself in some sort of physical danger. Start your romance with a hook to get readers interested in the characters and the story.
Your characters' actions will ultimately determine the direction the story will go. Focus most of your efforts on your hero and heroine. Ask yourself who they are at the beginning of the story. What are their likes, dislikes or habits? How do they need to grow throughout the story in order for their relationship to work? What are their feelings about entering into a relationship in the first place? The more you know about your characters, the more believable their relationship will be throughout the book. Remember that change and development are what make characters and their relationship interesting to readers.
Readers may want a happy or hopeful ending when they read romance novels, but relationships that develop without conflict or problems are both boring to read and unrealistic. Your conflict is what drives your plot. You may end up with several conflicts throughout your romance novel. There might, for example, be a conflict between the hero and heroine about an event from the past or a misunderstanding about the future. While dealing with this struggle, each of them may also face an internal conflict as they come to terms with the situation. Identify the internal and external conflicts that your characters will face as the story progresses.
After you have an idea of who your characters are, what conflicts they will be facing, and what direction your story will take, make an outline. This will serve as a guide as you write, but should no means be considered final. Feel free to make changes to your outline if the story takes a turn you did not anticipate. (Don't panic if this happens. Stories that take a life of their own can turn out to be the most unique and interesting.) Include significant scenes and events in your outline. Try to list them in the order you want them to appear in your romance novel, but don't hesitate to rearrange, add or remove scenes if you need to.