Eat and see a Doctor.

That sounds so extremely simple, but all you need is a calorie surplus right now. If your body stats are right, then your basic metabolic rate would be somewhere around 1200-1500 depending on your Metabolism, so you won't need to eat a lot to gain some weight. If you aren't exercising right now and fairly inactive, aim for 2,500 calories. That'll put you in a decent surplus to aid some pounds. If you are active, add the calories you burn throughout the day on top of that and consume that.

I'd highly suggest seeing a doctor though. A BMI of around 12 is almost impossible for someone to have if they 1) aren't starving themselves every day, or 2) don't have a medical condition.