Sir I am 19 years old and my weight is just 45 kg how can I increase my weight? And my height is 5 feet 6 inch. And I am not taking any supplement.

Improving weight. First of all you need to know if you have any medical problem causing you not to gain weight. ie. celiac disease, diabetes, malabsorption. If you are sure you have none of these, check what food you are eating, if they are good sources of calories. If you are very active you might be loosing more calories than what you take in. You can supplement your calories with high calorie diet .