It was something I'd wanted to do since I was about 14 years old--and, finally, I got up the nerve," says Debra Peterson, thinking back to the day when she went to a photographer and confessed her secret desire to pose for a Playboy centerfold. Our ingenuous 21-year-old Californian--she was born in Santa Monica and grew up in Rolling Hills--had no experience before the cameras; but, as you can see, she didn't need any. Her parents weren't exactly enchanted with her move--"You know how it always is with the baby of the family," says Debbie, who's the youngest of four children--but her boyfriend, a technical advisor to film makers, gave her new-found modeling career a quick boost by making a connection for her to do some TV commercials. It promises to be easier work than breaking in horses, which she used to do professionally as a groom and exercise girl for a thoroughbred trainer. She left the job about a year ago, after deciding that the money wasn't enough to make up for the risk of injury. Debbie's been riding since she was six, when her parents--like a lot of other people in Rolling Hills, a well-to-do suburb with plenty of trails--bought horses for their kids. When she was about 15, though, her parents split up. Debbie had to give up her horse. She stayed awhile with her mother, then with her dad, before striking out on her own three years ago. Now, in a sense, Debbie's turning back the clock; she's bought a thoroughbred of her own and she's keeping him back in Rolling Hills, which is a 45-minute drive in her VW from the Marina del Rey apartment she shares with her boyfriend. In addition to riding, Debbie also goes in for water-skiing, snow-skiing and flying. Obviously, her fun time is going to be limited as she gets more modeling assignments. And eventually she hopes to go into business: "I'd like to be a fashion buyer or something like that. So I'll most likely be going back to school in a year or two. Actually, I hate school--but everyone says it's necessary if you want a job that pays well." Right--unless you have some superb natural assets and an instinct for where to take them.