I am of age 18 and have a height 5ft 3inches. I am 45 kgs. Is that weight okay for this age and height? If not how can I increase it?

Diet and muscle mass. To gain weight you have to consume more calories than you are burning. Do this in a healthy way though. Don't choose junk or fast food. Keep a well balanced diet and make sure you are getting all the nutrients you need. Don't cut out exercise as it is very good for you. Weight lifting and other muscle building exercises can help you increase your body mass.