well i'm not sure what the perfect diet is because every doctor or health physician you go to says somethin different, ya know? but to lose weight i'd say that cutting the junk food helps, eating the proper servings of fruit and veggies will help. drinking water and milk.
for showin off ur abs, i'd say that requires some work. don't try it if ur not willin to get sweaty and tired. i would think that finding multiple ab excercised would be best. just doin crunches doesnt always work. u should find 2-5 different excercises that work the abs and make sure they work different places... lower, upper, obliques
here's a very basic principle: u will lose weight if u burn more calories than u take in. eating small breakfasts gives you no energy throughout the day and eating large dinners give u lots of calories that you usually don't burn off (unless u work out at night). so try eating larger breakfasts and slowly decreasing your meal sizes from there.