Crunches are the easiest most well known exercise to strengthen your abdominal muscles.
They're a lot like sit-ups, but instead of lifting your entire back off the floor, you only lift your upper back.
This isolates the rectus abdominus muscle (the six pack) without engaging muscles in your hips. This exercise also does not engage the deeper core muscles.
Its is better to put your hands on your chest, when putting hands behind the neck people tend to pull themselves up with their hand which causes neck pains, and a less effective workout.
Try to keep looking up at the ceiling. It can be tempting to move your head forward because it makes you feel like you're lifting further, but you're not. It's a bad habit that increases your chance of getting injured. One good way to prevent this is to always try to keep looking up at the ceiling.
Keep a slow pace on the way down Don't just fall back, the lowering down works your ab muscles as well.
Don't go too high The crunches are made to isolate the rectus abdominus muscle (the six pack), going to high will engage your hip flexors making the exercise less effective. Make sure your lower back stays on the floor.