I am 22, very slim, weight - 47 kg height 5.8 ft, what to do ?

See Dr., then eat...: If one is truly underweight (based on height, family background, and body type), a Dr. can evaluate for a hormonal, digestive, or other disorder. If no medical cause is found, then one can add weight by eating more calories of nutritious foods, while also doing weight-training to increase muscle. Proteins: egg white, chicken breast, salmon, ... Better fats: olive oil, peanuts, almonds, tofu, ....

Underweight: First effort should be made to rule out medical causes of underweight. If negative increase total caloric intake and increase resistance exercise.