Sit-ups won't give you a flat tummy. You need to lose weight to get that effect, the sit-ups may tone an already flat tummy....although crunches are considered to be better and more effective for toning the abdominal muscles.
Crunches are better and what i do, for two minutes (only 1 when i started) is lay flat with your arm almost like doing press-ups and lift your body up with only your stomach and back muscles, with a little support from your arms. It works, i've lsot an inch off my stomach since the end of january, and thats without dieting or any extra exercise other than biking to school and badminton once a week.
Whitebear some really effective stomach exercises at home are crunches, reverse crunches leg raises the "Plank" and "Sideplank" just google these exercises keep your meals small and lower your calories and you'll thank me before summer!