Just wait 20 years, I'm sure you'll have gained all the weight you wanted, and more!
Seriously, though, I wouldn't focus on gaining weight per se. Your doctor has said your weight is fine for your body. If you're interested in improving health, join a gym or buy your own weight set and begin a serious strength training program. Increasing muscle mass will
increase your weight (all else being equal). Don't necessarily expect to become a massive muscle-bound hulk or anything, that sort of thing is controlled mostly by genetics. But it's always a good idea to engage in strength training if you can handle it.
In an ideal world, consult a strength trainer and nutritionist to help you plan your exercise/diet plan. (If you're in school, this sort of thing may be available for free or at a nominal cost.)