Sit-ups can strengthen your belly muscles, but doing them incorrectly can hurt your back. Sit-ups should be done while you lie on your back with your knees bent enough for the soles of your feet to touch the floor. Place both hands on your chest and slowly raise your head off the ground. Then, slowly raise your shoulders around one foot and then lower them to the ground. Do this slowly ten times, rest a few seconds and then do two more sets of ten. After a week or two, this exercise will feel easy, so do it with a light weight, wrapped in a towel, held behind your neck or on your chest. As you become stronger, you can use heavier weights.
There's no need to do more than 30 sit-ups in one workout. To strengthen your belly muscles, you increase the resistance, not the number of repetitions. Keep your knees bent to protect your back. If you do a sit-up with your legs straight, you place a great force on the iliopsoas muscles that increase the arch in your back, which can damage the ligaments and joints. If your belly muscles are weak, you are likely to arch your back excessively when you sit up and increase the chances of injury. If you are doing sit-ups to flatten your stomach, you need to raise your head only about one foot because going higher than that uses the quadriceps muscles in the front of your upper legs, not your belly muscles.