I'm about to be 18 and I just weight 103 , I WANT to have meat on my body. What should I do? What should I eat ? But still be healthy

I've been in the same boat. Lift weights 3 or 4 times a week, eat lots of healthy, high calorie foods, get enough sleep and drink lots of water. Don't do too much jogging or you'll burn too many calories to bulk up. Your metabolism is very fast at 18 but should start to slow by 30 and you'll find it easier to gain weight then. Look on the plus side, you can probably eat whatever you want and many people wish they could do the same. Good luck.