Im 15 5'4 weigh 96 pounds , I can't seem to gain weight.?

I don't know how to gain weight.. I was 105 pounds , because I started eating allot and eating only healthy food barely any soda for a few days but in like 1 week I weigh 95 pounds.. what am i suppose to do? I need to gain like 30 pounds to be average weight (im underweight atm) am I just suppose to eat tons of junkfood or what?