Hi im 19 years old and underweight as im 39 kgs. my weight is constant since the past 4 years please any recommendations?

Start with checkup: You should first start with a review of medical history & a physical exam. You likely need a variety of tests to identify any medical conditions or genetic issues that influence your health. A mental health screening would be part of that process. These are likely to identify specific needs that need attention before you can put on weight in a healthy way.