✔ 最佳答案
首先, If I were you, 是 I would... 而不是 I will...
至於 go on a diet, 字典的解釋是 to eat only certain foods or a small amount of food because you want to lose weight. 即已包含 to loss weight 的意思. 所以不用重覆.
全句是: If I were you, I would go on a diet.
參考: Oxford Intermediate Learner's English-Chinese Dictionary