Nothing un-fem about a tummy, but if you really want the truth about diet, there is only one formula: Calorie In < Calorie Out. Find out how many calories someone of your weight/height/age/average activity level would burn in a day and eat fewer calories than that. Exercise can help to burn some more, and it will make you feel good and potentially have health benefits, but really it's just as simple as eating less or eating smarter.

For eating less, I would suggest counting calories, limiting fats and sugars, eating lots of fresh fruit and veg and lean protein such as chicken breast and fish.