Heya, I'm a rather active 5"5, 21 year old female who's currently on a 2,900cal diet!! Having suffered anorexia nervosa/athletica for 5years, I've finally owned up to the fact i'm underweight and am trying to gain 1lb/week (thats a healthy amount for each week) to get back upto 7.5stone. Best way to go about gaining weight (from 3hospital admissions experience and my dietitians' advice) is to record for 1week how many cals you eat (don't have to be obsessive about it, just read the labels), jot down how much you need to MAINTAIN your weight, then add on 500 more to that (need 3,500 more cals/week to gain 1lb, on average.) If you've a fast metabolism/slimness runs un your family, you might even need more than 500 extra. Since you're a guy, and still pretty young (your metabol will be going like the clappers right now) this will probably apply to you.
Then get your butt in the gym and push some seriously heavy weights, to make sure you gain muscle and not fat! 'Course, you will gain some of the latter (comes with the territory, can't be avoided) but you want the majority to be muscle, to make you stronger and not a lardass!
DO NOT go anywhere NEAR so-called build-up drinks/powders, they're an expensive waste of space and taste like crap to boot. Trust me, I was a complete ignoramus and tried a few, BLEH, what a waste of cash and time. You're better off filling up on nutritious stuff. Check out the internet - go on Google, type in "healthy weight gain diet" or sommat like that, it brings up loads of ideas for healthy snacks/meals, etc. Not to mention muscle-training tips.
Hope some of this stuff helps mate. Good luck and post me a line to let me know how you get on, or if you need some help.