To gain weight, try eating a small meal every 2-3 hours and consciously limit your daily activity. Try to stick to simple foods like oatmeal, brown rice, cooked chicken, or lean ground beef (drained).
People tend to get fat not because they eat insane amounts, but because they eat empty calories (pop tarts, soft drinks, beer, most microwavable food). When your body isn't getting the nutrients it needs, it assumes that it's starving, so it starts storing up fat.