First weigh yourself and be doing it regularly then start to lift light weights since they make you strong. Exercise is the best way for you to get strong without gaining weight. Watch your diet avoid fats and take fibers. See more details at http://www.ehow.com/how_4880858_stronger-putting-weight.html.
Well actually to get strong means building muscle but fear not muscle weighs more than fat also a diet high in protein low in carbs helps and you should eat every two to three hours to keep your metabolism up witch in turn will burn fat and not store it for energy