My bmi is 23 but my belly and chest are fatty and saggy. why is this so when i have normal weight and how to flatten and tight my belly ?

Depends...: You may be someone who is not carrying a lot of muscle and therefore even if you're at a healthy weight, you could still have a relatively high percentage of body fat. In order to improve your body composition (ie your muscle to fat ratio), you should eat a relatively high protein, low sugar, low starch, low grain diet and do not over eat. Also, you need to work out with weights and do cardio.