you just have to up your calorie intake and eat a decent meal every 3 or so hours and do some exercise. if you feed your body more and give it a reason to need all the extra carbs and shit then you will put on weight.
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5 months ago - 5'9 112lb's & now i'm a half decent 6'0" 155lb's

i used to be exactly the same. i used to eat 4-5 big meals a day and even snack and i either stayed the same or lost weight, even if i only sat around on my ass doing nothing. now i eat 7-8 meals a day, super high carb & protein foods. + go to the gym and hit the weights monday-friday and just rest weekends.