Let me break your questions into two parts and answer them.

How do I reduce belly fat ó You just canít reduce belly fat alone. You have to reduce fat from the entire body and thatís how science works. Spot reduction of fat is not possible and if any gym trainer who told you that doing crunches or running on a treadmill will help you burn belly fat, he is either wrong or he is lying to you. Even if your nutritionist is telling you that you will reduce belly fat by warm water , honey , lemon , green tea etc, may be you want to check where she got her certification.
How do I reduce fat fastly(fast) ó Again , not a good idea. You might loose some weight by eating in deficit. Eating in deficit for long is not advised as it will screw up your metabolism and make you weak. Also , when you try and come back to your normal routine, you gain back the lost weight and even more. Ainít a good idea!!
You have to create a structured diet for yourself and follow it increasing your calorie intake gradually and also the intensity of your workout at the same time.Your body gradually adjust with your new shape and size and makes it your new home.I haev answered couple of answers related to healthy lifestyle earlier which might help you.