Summarize:

If you're a relative beginner, start off by working out three days a week, and add a fourth over the course of a few weeks. Spread your most intense workouts out over the week so you'll have plenty of recovery time. If you're more experienced, you can stick to a more demanding, five or six day schedule. It's best to check with your doctor before starting a new exercise routine, especially if you're a teenager (with growing muscles and bones), or if you have any chronic medical conditions. A personal trainer can help you come up with an exercise program based on your goals and level of experience. Plan your meals, control your diet year-round, and keep track of how many calories you consume daily. Your needs will depend on your age, sex, and fitness goals. Monitor factors like your weight, body fat percentage, and strength gains, and adjust your diet accordingly. For example, if you want to reduce your body fat percentage, stick to healthy fats like olive oil and omega-3 fats (salmon and halibut are great sources), and add a cardio day to your schedule. Don't forget nutrients when you're counting numbers like calories, protein, and carbs. Reducing calories gradually is necessary to sustain the fat burning that makes muscles visible, but don't lose sight of your overall health. If you're allowed a set amount of a type of food, meet that requirement with a rich source of nutrients. For example, if you're allowed 200 grams of carbs per day, don't squander them on sweets that contain no nutrients. Instead, go for nutritious carbs like potatoes or berries. Don't rely on multivitamins, and don't make skipping meals a habit. There's only so much you can accomplish without learning from an experienced trainer. When you're just starting off, work out with a trainer at your gym. As you break into professional modeling, get a trainer who has experience with your area of focus. For example, competitions and print require different physiques. If you compete regularly, you'd want a trainer who can help you achieve and sustain a contest-ready shape. Even if you've achieved a good physique on your own, a trainer can help you set your goals even higher.
Develop a workout plan based on your experience. Keep track of what you eat. Don't cut nutrients when you cut calories. Get a professional trainer.