If i stop eating sugar and do crunches, and other stomache exercises along with swimming will i lose fat?

Im only 14 and weigh 115 and im 5`2. i just want a flat stomache. & I dont want to do a really demanding diet that you have to cook weird recipes and ect. cuz i dont grocery shop. haha. We usually eat the basics like chicken with green beans and potatoes for dinner. and sandwhiches for lunch. Cereal for breakfast. and sweets in between. If i cut out the sweets and soda will it help? with swimming and abb excersises?