A person can be underweight due to inadequate intake of calories or some underlying illness. Remember the following points to increase your weight -
• Have small frequent energy dense meals in the whole day. Take at least 5-6 meals in a day.
• Improve eating habits and focus both on quantity and quality of food you consume.
• If you are not gaining weight due to some underlying medical condition like anorexia nervosa then identify that first and treat it.
• Include foods like nuts and oilseeds, high carbohydrate foods, milk and milk products, pulses, sprouts, eggs, chicken, fish or meat in your daily diet.
• Regular exercising will also help to build muscles.
For acidity remember the following tips -
• Avoid starving or keeping large gaps between your meals and avoid over eating.
• Also avoid oily foods, too spicy foods, strong tea or coffee. Don’t have tea or coffee alone (even if it is mild); at least eat a couple of crackers to avoid acidity.
• Eat meals at least 2-3 hrs before sleeping hours and relax and sit after eating a meal (avoid lying down immediately after a meal).
• Avoid taking any kind of stress, mental or physical as far as possible as this would worsen your condition. 
• Completely avoid alcohol, smoking and tobacco.