Hi there...I can understand your concern. It would have been better if you had mentioned your height. So that we could have calculated the appropriate weight for your height. Nevertheless it is important to gain muscle mass and not fat.as you know few people have very high basic metabolism rate. It will be difficult for them to gain weight. Also one should remember gaining weight and losing weight is a very gradual process. It will a lot of time, patience and determination. You must follow a healthy diet and exercise schedule. Start having food rich in proteins like pulses, soyabeans, vegetables, fruits, egg whiteboiled , boiled chicken. Regarding exercises, start cycling or swimming or gyming, whatever you are comfortable with and enjoy most. You can also visit a dietician and endocrinologist for further queries.