wintonyk is right. You really need to eat a lot.
eat until it hurts sometimes...and if you are skinny, eat some junk too...burgers may build fat...but also decrease metabolism.
I used to be in the same boat...albeit not that bad...but I started at 175-180lbs at 6' I got up to as big as 245-250...but it got hard on the feet and lower back to be hauling my large ass around all day at the shop...
I am back down to 225-230 and happy.
The key is TONS of food....always be eating. In the beginning, eat anything you want...and lots of it. I hope you make a fair bit of money, because you are going to need it.
Oh, and one more thing....shrugs, dead lifts, and squats man. it really spurs muscle growth. I know they look like gay exercises but they work.
Try it for a couple weeks, and you'll see.
1999 NBS Silverado 4x4 awaiting an extensive build.
6" BDS Lift is in. 37" mud grapplers. Built trans(by me) 4.10's, shaved ls6 heads, comp cam. More to come.