Losing or gaining weight is all about calories. If you want to lose weight, you consume less calories than you burn throughout the day. You want to gain weight then consume more calories than you burn throughout the day. It's as simple as that. 3500 calories = 1 lb. If you're overeating by 500 cals a day, then you should gain a pound a week.
The problem with skinny or fat bastards who complain about "I can't lose weight!" OR "I can't gain a pound!" is because you're not paying attention to what you're eating. If you actually counted everything you consumed...you would eventually be able to pinpoint your problem. As a fat bastard who "can't lose weight" I know its because of what I eat. If I actually tracked what what I was eating and burning throughout the day, I'd easily be able to predict where I'm going to be tomorrow.
Losing or gaining weight is 70% diet. The rest is your lifestyle (how often you exercise, what you do, job, etc.). Lay a foundation and the rest will come.
Better Body Journal » Blog Archive » Laying the Foundation - Part 1
I present you skinny bastards with this. Its from my favorite weightlifting website out there. It goes in depth about pretty much what I just talked about.
Testosterone Nation - The Skinny Bastard Diet