How to diet and not become weak?

Eat enough: Enough protein, calories, minerals, fluid, vitamins. Cut out junk, simple sugars, minimize alcohol. Small frequent meals can help; don't skip breakfast; protein at every meal.

Eat right: Diet does not mean starve, in fact starvation diet is the worst you can do and will never provide you with long lasting results. You need to eat sensible amount of calories depending on you basal metabolic rate, and keep diet well balanced. (include all main nutritional groups), good diet will teach you how to eat right and in moderation and allow you to keep the weight off for a long time.