According to your height your ideal body weight should range form 75-80 kgs. That means you need to gain at least 17-22 kilos more to be the least acceptable weight. Set a goal to achieve at least 3-4 kg in a month.
Follow these guidelines for gaining weight –

• Gradually increase your whole day’s caloric intake. Have small meals at frequent intervals (have at least 6 meals in a day).
• Consume energy dense foods such as mango milkshake, banana milkshake, nuts (soaked almonds, groundnuts, walnuts, cashew nuts or any others according to your choice).
• Consume dry fruits like figs, dates and raisins 4-5 number in a day.
• Avoid consuming water before meals.
• Exercise to increase the muscle mass and also to increase your appetite.
• If you have any emotional disorder like anorexia nervosa try to recognize it and treat it first.