Last year, I was barely 56 kg. Now, I am currently 70 kg. If you
can't do the math (mass?), that means I gained 14 kg in less than a
year.
Eat a lot. Excercise without eating is like... well I don't know,
but it's not very good.
I ate 3 full heavy meals a day (make sure you don't skip breakfast
or anything like that). Another good idea is always to eat at more
or less the same time. This will help your body absorb the food
better. Drink milk. I downed at least a litre of milk a day
(sometimes two!), on top of my 3 meals.
Exercise. Do weights if you want to build mass. Go for "power":
more weight, less repititions. (The opposite is "edurance": less
weight, more repititions.) This will build up mass.
Suggestions:
for max power: 10 reps, rest, 10 reps (heavy weights)
medium: 15 reps, rest, 15 reps (medium " )
endurance: 20+ reps, rest, 20+ reps (light " )
Don't worry too much about becoming too "big". Body builders do
sh*t like this for years and take steroids and stuff. If you're
like me, one of those chronically skinny types, eating a lot,
sh*tting a lot, and excercising a lot won't hurt you at the very
least.
Also, don't take any "mass building" formulas. No need, if you eat
properly.