My age is 20yr and my weight is 50kg iam looking very thin what should i can do?

Develop what u have: You may be light-framed which is a plus especially later in life. You're almost grown and this is a chance to put on some muscle while keeping off the body-fat. Do some resistance training even if you are a slow-gainer; some creatine as a supplement may help you get bulk. Doing aerobic fitness will help as well. Glad you are willing to work for the body you want. Best wishes.