There are tons of books on nutrition, which is seriously all you need to know

As a general rule, a person shouldn't drop under 1200 calories while 'dieting'.
If you fill your diet with vegetables, fruits, whole grains and small amount of fish and fowl - and no sweets (that is the hard part!) - your body will naturally drop weight. You should never allow yourself to become famished - instead eat every 3-4 hours. But something like an apple or a bit of celery with a spoon of peanut butter for the protein.
My favorite is a cut up apple dipped in low fat sugar free vanilla yogurt.
Don't fall for any eating plan that tells you to eat all protein and no carbs. That is rubbish! Your body needs carbs too. Just the right kind of carbs. Fresh veg is the way to go.
Good luck. 'Dieting' is hard.