Plenty of 13-year-old boys and girls want to add muscle for sports, or to look or feel better about themselves. However, teenagers must be cautious when starting a workout routine. The growth plates in kids' bones are still growing and they should not put undue pressure on them, according to the Mayo Clinic. Therefore, it is best to either do body-weight resistant exercises like push-ups or lift relatively light weights.
Get used to resistance training. Do body-weight exercises without weights, such as push-ups and squats. Perform exercises for all areas of your body like chest, shoulders, back, arms and legs. Work out your entire body each workout, and perform the exercises three days per week.
Do push-ups. Place both hands on the floor and support your lower body on the balls of both feet. Inhale as you lower your chest to the floor, then exhale as you elevate yourself off the floor. Do three or four sets of as many repetitions as you can.
Do squats. Bend your knees at a 90-degree angle. Raise your arms out in front of you for better leverage. Do three sets of 10 to 15 repetitions.
Work out your shoulders. Using a length of rubber tubing for resistance, do side lateral raises. Wedge the rubber tubing between a closed door and the jamb. Hold one end of the rubber tubing with your right hand and, while keeping your arm straight, raise the tubing up to shoulder height. Lower the tubing back down. Do 10 repetitions and repeat with your other arm.
Start weight training after your body gets used to the initial soreness of resistance training--six or eight weeks later. It is best to train three to four days per week, working the upper body one day and lower body the next day, according to bodybuilding.com. Rest each muscle at least two full days in between workouts.
Train your larger muscles first each workout, such as chest the day you work the upper body, and thighs the day you train the lower body.
Do exercises like bench press for chest. Select a weight that you can lift 10 times for your warm-up set. Stay between six and 10 reps, adding 5 or 10 pounds to the bar after each set. Lie on a bench and lift the barbell up and out so it is directly above your chest. Lower the weight to the center of your chest, then press it back up. Do three sets of this exercise.
Do squats first on leg day. Place the barbell across the meaty part of your shoulders and perform the exercise as previously described.
Take monthly measurements of your muscles and body weight to track your progress.