I'm 21 yrs old but still so thin (about 49 kg). i usually eat a lot but there is no increase in my weight. what might be the problem and how can i solve it?

Let it be: If you are otherwise healthy, but just skinny, let it be. With age you will gain weight and then you will look back at this time fondly, when you could eat and not gain weight. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.