I totally agree with OR, especially at 18 going on 19 you can gain ridiculous weight if you want to.
However, "weight" is a relative term. You say you want to build muscle, but simply adding pure muscle is only going to be around 5-7 lbs a YEAR. But if you want to get HOOOGE, well eat like you stole something and you'll easily get 50 lbs heavier because a decent dose of fat will come along for the ride.
My point is answering your question really depends on your goals. You're damn near 6' and a buck fifty. I'm 5'7" and damn near 2 bills. Within reason, I don't care about my actual weight on the scale, I care about the weight I can lift. But obviously I'll never be a fitness model. However you could if that is what you're thinking.
So, are you gaining weight to simply fill out some muscle mass without losing your abs, or do you want to stuff yourself and be a lot stronger, or something in between?
Gaining "weight" is easy. Meeting a goal is a bit harder. Explain your goals and answering your questions is easier.
I am Anton Zdravko Martin!
Best meet lifts: Sq 150 Kg (330 lb), Bench 120 Kg (264), DL 160 (352) @89 Kg (197)
Best gym lifts: Sq 375, Bench 280 (pause), DL 385 @205 or less
Goals beyond my wildest dreams, 600 Kg raw total, 200/160/240
Hack away at anything which isn't essential. Do what you love, and do it often. Fazc.
Everything competes for recovery so more assistance is not always the best idea. miked96