While aerobic exercise strengthens your heart and lungs, strength training strengthens your muscles and helps decrease fat and increase muscle tissue. Strength training involves using your muscles against some form of resistance, such as barbells, dumbells, machines like Nautilus, or even your own body weight. You can also make weights out of things like empty milk jugs filled with water or sand. Weight training makes you look toned and feel strong. Most girls will not bulk up with weight training, so you don't need to worry about bulking up like a guy if you weight train. However, strength training does carry some risks, so it is important to do it correctly.

You should strength train two to four times a week, giving each muscle group a day to recover between workouts.
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Your school gym instructor, coach, or health club counselor is qualified to get you started on an appropriate strength training program. You can tell them your fitness goals and they will identify the right exercises just for you. Be sure to check with one of them before trying to use weights yourself. If you decide to use "free weights" (dumbells or barbells), be sure you have a friend or spotter nearby at all times. Dropping those weights on yourself would not be good for muscle building!