Crunches are used to build the upper area of the abdominals as well as strengthening your core.
Lie flat on the ground with your back firmly pressed against the ground. Your knees need to be at a 90 degree angle and your feet should be firmly planted on the floor. Place your hands on the sides of your head and keep your elbows in. Now, contract your abdominals and pull your shoulders off the floor. Your lower back should remain on the floor and your shoulders should raise 3 to 4 inches off the floor. Squeeze your abdominals when you reach the top position. Then, slowly lower your shoulders back to the ground. Repeat until you have completed your set number of repetitions.
Do not lock your fingers behind your head. Also, do not pull on your neck with your arms; this will cause you to strain your arms.
You want to pull your shoulders straight up off the ground, try not to curl up. This will focus all of the tension to your abdominals.