An excellent exercise to work the abdominal muscles is called Toe Touchers. You do not need any equipment to do this exercise (though you can add weights eventually) and it utilizes your own body weight.
Lie on your back, on the floor and elevate your legs up into the air. Make sure you keep your lower back pressed flat into the ground and do not arch. Pull in your abdominals. Exhale and crunch forward reaching your hands upward to touch your toes. (Touching any part of your foot or shins will work!) Please remember, don't pull your chin down towards your chest. Keep your head in a neutral position.
In this section, you will find a collection of various exercises as well as tips and information on proper form.