Don't set your heart on loosing weight fast.
Anyway, 1,200 is a good number. Anything technically below 900-1,000 is starving yourself. You have to realize that once you go below a certain amount of calories, your body goes into starvation mode and instead of losing weight fast, you'll actually retain every ounce of fat and carbs you eat, your metabolism will slow, and you will end up gaining.
Also, ditch the "exercising sometimes," thing. 5-6 days a week for 30-60 minutes is ideal. x3