How can i gain weight?

ight first of all im 14 years old, 5'3 and 100 pounds, im a skrawny lil mother f*cker, and wanna gain weight, i wanna be at least 125 minimum! i eat ALOT. and for like 2 months i had jack in the box i pretty much had jack in the box everyday after school. i also ate carne asada burritos that were huge, i just ate alot period. and i dont gain weight whatthefuck? i either lose 1 pound or stay the same, i work out but not really running but now i have to cause im takin freshman football, but all i used to work out was my upper body like bench pressing and dumbells etc. well how can i gain weight please help me thanks ;D