Imagine your protagonist. What does she look like? Does she have flowing red hair that she ties back with a bandanna so she can ride her favorite horse, or is she a blond librarian? Where did she go to school? What is she doing after she wakes up in the morning? If you like, take notes about her, or write a mini biography.
Your protagonist will help you choose your genre of romance, which makes framing the story easier. Romance Writers of America classifies romances as contemporary (set after 1945); historical (set prior to 1945); inspirational (with religious or spiritual beliefs at the story's core); paranormal (with fantasy or otherworldly happenings in the plot); suspense (a mystery or thriller); young adult (geared toward teen readers); and Regency (set in the Regency period of the British Empire, generally from 1811 to 1820 -- think of Jane Austen). Picture your protagonist from Step 1. Try different scenarios and see which possibilities appeal to you.
Create the love interest. What does he look like? How do they meet? If you're comfortable with your protagonist and her genre, this should be simple. Imagine his typical morning. Does he have a valet arranging his clothes? Is he running five miles through the park with his iPod before making coffee? Jot down ideas.
Pick names for your pair of lovers. Unless you have a name that you absolutely adore, hold off on choosing a name until you can picture both of these people fairly well. Some names are better suited to certain genres. A Regency-era girl, for instance, would be named Elizabeth or Margaret, but not Amber. Try a baby-name book to learn the origins and meanings of names.
Outline the big problems your lovers face. You don't have to follow it strictly, but have an idea where you want your story to go. Make notes about what might help them overcome these obstacles.
Start fresh. In "Hooked," Les Edgerton writes that a bad beginning will stop readers cold -- so avoid opening your novel with all that wonderful background you've created for your main characters. Place your protagonist in a tough time when your story opens: a minor crisis that sets up her personality and the larger problem she's going to have to solve.
She doesn't have to meet her love interest in the first five pages -- romance readers will be patient for that -- but don't spill her whole biography at the start, either. Drop out little morsels along the way.