I weigh 42.2-45.5kg, am an 18 year old female, and would like to put on some weight and i can't seem to be able to. advice?

Talk to your doctor: You may be a normal weight for your height and body type. Check with your doctor to find out if this is a normal weight for you. If your doctor thinks you are underweight, she can determine if there is a medical problem or if you have any nutritional deficiencies. It may depend on your exercise level, your heredity, and what you are eating.