I want to gain weight as i am very lean and thin. so please guide me.

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,...