I do not think this question is one that can be answered here, especially as you have referred to the fact that you are looking for medicines to increase your weight.
You should contact a certified medical professional. There are many reasons why people do not put on weight; they could be linked to a hormonal imbalance, deficiency of some nutrients, a wrong diet, etc.
A medical professional will be able to first determine if everything is fine with you and then refer you to a nutritionist who will be able to analyze your diet and your daily activity to advise the changes that you need to make.
You would most probably be asked to eat a lot of healthy foods, increase the intake of calories, maybe take nutritional supplements, and the alike.
Do not ask for advice from your friends and others without an adequate knowledge on this subject. Taking the wrong pills to achieve what you want to do can be injurious to you.