@ Lavanya,
The best way to increase weight & muscle is increase protein intake. Protein is the muscle builder activator, I highly suggested to have proteins rich foods such as, eggs, fish, chicken (skinless preferred),dairy& dairy products , dals , sprouts , Soya beans. One should get full of rest, thus sleep well for 6-7 hrs. some healthy drinks like apple juice, orange juice etc. Drink 2% or 1% milk & eat egg white daily in ur breakfast. This combination surely help you to increase ur weight as well as muscle.