Re: Weight Gain
In order to gain weight, you will need to be in a modest positive energy balance, ie. eat more.
In terms of diet, I would increase caloric intake by incorporating at least 3 snacks and 3 meals a day. Try your hardest not to skip meals, and make each meal count.
Examples of some high-calorie foods/snacks that have healthy unsaturated fats: peanut butter, cheese, nuts (eg. almonds, walnuts), seeds, dried fruit (eg. apricots, figs, prunes, dates), protein/energy bars, fatty fish (eg. salmon, sardines, mackerel -- a good source of omega-3 polyunsat. fat, as are flaxseeds), avocadoes, granola. Monounsaturated fats from canola or olive oil are also good.
Examples of some high-calorie beverages: smoothies, 100% fruit juice, 1% or 2% milk.
In terms of exercise, weight lifting (together with adequate calories, esp. dietary protein) will help with increasing mass and the type of mass you gain.