I have no idea what to do?

I'm 16 and I weigh 120 pounds. I pretty skinny,but im built skinny,if thats possible,if you know what I mean. I work out for like 30 minutes,everyother day,and i keep losing wieght and its really hard to gain weight if born thin.what should i do or take that will make me bigger in fat,bec i just usually sweat it off in like 5 sec