Don't start bench-pressing yet. First, you need to know what you're doing.
Consider hiring a trainer to get you started. Look for a trainer who is certified by a reputable group, such as the American College of Sports Medicine or the National Strength and Conditioning Association, says Avery Faigenbaum, a professor in the College of New Jersey's health and exercise science department.
Pick a trainer who is knowledgeable and fun to work with, Yuam says. He recommends splitting the cost with a few buddies.
Yuam's fitness video, The Werewolf Workout, features people of different heights and builds doing the same exercises to underscore that machine settings must fit your body size to prevent straining.
Pediatric sports medicine expert Andrea Stracciolini, MD, of Children's Hospital Boston, says poor technique and poor supervision are top causes of injury for teens who strength train.
Yuam describes examples of exercises that helped Lautner. With each exercise, reps depend upon fitness level.
Swiss-ball plank. Get in plank position with your toes on the floor and forearms and elbows resting on a Swiss ball. Your back must be straight. Hold for 10 counts. This works the transverse abdominals.
Reverse crunches. Lie back with your palms on the floor next to your hips. Bend your hips and knees so both are at 90 degrees. Raise your hips and crunch them inward. Usually, hold for five counts then slowly lower your legs until your heels gently touch the floor.
Planks to pikes. Start in face-down plank position. Breathe while slowly raising your hips upward -- using your abdominal and gluteus muscles -- to a V. Then, lower back to plank position. Hold for a moment at the top and at the bottom positions.
Hanging leg raise. Hang from a chin-up bar (or use lifting straps) with your feet together and knees slightly bent. Lift your thighs to your chest as your knees bend comfortably. Hold and exhale. Slowly lower your knees to the starting position while inhaling.
Prone cobra. This helps with core stability. Lie face down with your legs straight and arms next to your sides, palms down. Contract your stomach muscles and those of your lower back, and raise your head, arms, chest, and legs off the floor while rotating your arms so your thumbs point to the ceiling. Only your hips should be on the floor. Try to hold the position before resting. "Yoga and Pilates have versions of this," Yuam says.