Hi there....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 white, boiled chicken. Regarding exercises, start gyning, cycling or swimming, whatever you are comfortable with and enjoy most. You mist build your muscle mass to gain weight. You can also visit a dietician and endocrinologist for further queries.