If you're not sure what a healthy weight is, check out a Body Mass Index calculator (there are several ones online, a good one is at http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm).
This is the ratio between weight and height, and it gives you an idea what you should weigh. But remember that weight depends on a lot of things, like genetics and if you have a large or small frame. If you're really worried, ask your doctor at your next visit if your weight is healthy and ask for suggestions on how you can gain or lose weight to become more healthy. A lot of doctors are afraid to raise the topic unless the patient is REALLY under- or overweight, because it could hurt someone's self-esteem and cause eating disorders, so make sure you tell them that you want an honest opinion and suggestions on how to become more healthy by losing/gaining weight.