HELP!!! how can i gain more weight?

im 15 years old.ive been eating alot,i eat alot of junk food and sometimes healthy food,but i dont know why i dont gain weight what should i do?do i have anemia?i think have anemia because everyday i have fatigue and my nails are kind of brittle,sometimes when i get up from bed or a seat,i feel really dizzy and then i start seeing black and like stars,im VERY SKINNY,im so skinny you can see on my chest and my ribs show,im 111 lbs and 5'7',in school gym class,i ran really fast and after that,my chest and my throat was hurting really bad,and sometimes im light headed,and im also a little pale,and my leg muscles hurt and i cant walk right,people say i walk like a penguin,how can i solve all this!?!?!? PLEASE !!!