I thought the same thing about me. I'm 6'3 and used to weigh 145 lb so a real life skeletor.
I started hitting the gym everyday 5/7, eating protein every 2-3 hours and gradually gained 50 pound of muscle. Now I lift more than most guys in the gym, can lift my gf on one hand, have way more testosterone and I'm willing to and will fight anybody anywhere (even if I don't stand a chance).
Here is my advice. Buy all the chemicals namely weight gainer (muscle building is a highly energy demanding process so just protein won't be enough), get some creatine, BCAA, and ZMA plus vitamins. Eat a ton of meat (avoid chicken and dairy). Eat your greens too.
Do squats, deadlifting, dips, benchpress, pullups. Fuck cardio.
You'll be a muscle building machine in no time.