eat lots of protein enriched foods and work out as often as you can, making sure that you don't over-exert every day. you should have vigorous workouts once every three days or so and some less strenuous workouts on between. sex is also a really good workout too, but i would not condone it for your age.
Crunches and bicep curls on an exercise ball. Balancing your weight will help you with your core and it will make the crunches more complete. The exercise ball will work on all of your abdomen instead of just the upper abs.