Changing your eating habits for a healthier diet isn't bad. Just make sure that you eat all the necessary foods to keep a good and healthy milk supply if you plan on breast feeding. Just eat healthier food choices and don't over eat, the baby weight will melt right off.
It depends on the kind of diet, if you mean cut out soda, fast food and too many sweets then youll do good, but you cant cut back on healthy food, complete meals or skip meals if you want to breastfeed