It's not so much about what you eat (well avoid processed junk) but more to do with how many calories you consume.
A rough guide I use is stone converted to lbs x 14 to work out daily calorific requirements.
So for arguments sake, lets say I weigh 12 stones, thats 12 x 14lbs which is 168lbs, great so times that by 14kcal, basically we're saying each lb of body weight will use 14kcal...so thats 2352kcal needed to maintain a body weight of 168lbs.
In which case if I want to lose weight I'd eat at a deficit, therefore take 500kcal away from 2352kcal, so 1852kcal would be consumed daily to lose weight. Adjust your calorific intake as you lose weight and go from there
There are loads of other methods to work out how many calories the body needs, I've just given you an example of what I use, so do some research and go from there
To keep an eye on your calories I highly suggest MyFitnessPal, awesome app that'll count your calories for you!
As for the type of foods you consume, try to keep you diet clean & lean! So plenty of protein, moderate carbs and moderate fats! Avoid processed foods, sugary treats and alcohol, that along with some exercise will work wonders.
Also forget spot reduction, doing a million sit ups won't magically make your belly disappear
It's a game of cals in -v- cals used.
Goodluck!