I'm 18 years old still my weight is 42 kg only. I had tried many supplements but nothing happened.

Question

You can start with mild walk or muscles strengthening exercise followed by moderately rich protein stuff during the day. You can add milk and other dairy products in your routine. Seasonal fruits and vegetables will help you more to improve metabolism rate and increase weight. Add more of liquids and beverages during the day that will help you for the detoxification of the body. Take frequent meal at every 2 hourly. Make a record and accordingly further we can do the required changes.

Swarnshikha Sharma · Accepted Answer

You can start with mild walk or muscles strengthening exercise followed by moderately rich protein stuff during the day. You can add milk and other dairy products in your routine. Seasonal fruits and vegetables will help you more to improve metabolism rate and increase weight. Add more of liquids and beverages during the day that will help you for the detoxification of the body. Take frequent meal at every 2 hourly. Make a record and accordingly further we can do the required changes.

Karuna Chawla · Answer

U can try this -
1. 1. Don't take tea empty stomach. Eat something like a banana (if you are not diabetic). or any seasonal  fruit or soaked almonds and a glass of water first thing in the morning (within 10 mins of waking up). No only biscuits or rusk will not do. 
2.  Don't drink water before meals. This can fill your stomach and make it harder to get in enough calories.
3. Take your breakfast every day. Don't skip it. U can eat whatever ur mother or grandparent eat in bfast. I mean to say whatever is ur traditional food. If punjabi eat paratha, if belongs to south then take idli/ dosa etc.
4. Have light meals every 2 hours (in addition to your breakfast, lunch n dinner) e.g. Nariyal paani, chaach, a handful of dry fruits (eat whole fruit not juice),  a handful of peanuts, seasonal fruit, a cup of curd/milk, A cheese cube/slice/ banana shake etc
5. Finish your dinner at least 2 hours before going to sleep. Take simple food like rice n dal in dinner.
7.Take a lot of green vegetables n fruit. 
8. Drink lot of water	
9. Everyday preferably sleep on same time
10. Maintain active life style.  Exercise To Gain Weight Swimming:		
Jogging:					
Lunges And Squats:		
Bench Press:				
Deadlifts:				
Yoga:				
Pull-Ups:				
Upright Barbell Rows And Dumbbell Shoulder Press
Lifting weight is important too, so that the additional calories can be used to build muscle instead of just adding fat.
11. Don't smoke. Smokers tend to weigh less than non-smokers, and quitting smoking often leads to weight gain.