Sit-ups are a great exercise to strengthen your entire core.
Begin by lying flat on the floor with your knees bent so that your feet are flat on the floor. You may want to keep your feet under a stationary object or have a partner hold your feet down. Place your hands behind your head and clasp your fingers together. Begin the movement by raising your torso off the floor by bending at your waist and squeezing your abdominal muscles. You want to continue this movement until your torso is nearly straight up or perpendicular to the floor. Squeeze your abs at the top and then slowly lower your torso back to the floor. Repeat this movement until your set is complete.
Do not pull on your neck with your arms, it could cause injury. If you have a tendency to pull your neck up, do not lock your fingers together and place your hands on the side of your head.
You can make this movement more difficult by holding a weight plate on your chest. You should cross your arms over top of the weight plate and hold it tight to your body.