You're 13. Don't do anything until your body has matured. Just enjoy your youth.
Hiim seventeen im 5'7 and weigh 100 i also have a hard time gaining weight because I have a very high metabolism. In the last year I have been able to gain some weight by doing some things. I went from 90lbs to 100lbs by eatings protein and even drinking protein shakes called insure. When you want to gain weight you need to remember to eat healthy still do not gorge your self on junk food. Try to gain some muscle weight, dont over do the protein to much can be bad for your liver ect.