In order to gain weight you need to consume more calories than you expend. Use this Caloric Calculator to figure out how much you need daily.
As for the the frequency, you should have 5-6 meals, and try to space them out evenly. Maybe after breakfast, eat some fruit and a protein bar. If you eat 3 big meals, YOU Wont gain anything at all. I am in the same boat as you, I have a very low BMI, and a quick metabolism. Unless you start going to the gym ( or working out at home), you will not gain any muscle. You desire to eat will also be increased after workouts. In fact, the Very best way to gain weight is to eat right after your workout. Some people feel that they are cheating themselves if they eat right after, but studies have shown that it is actually very important to do replenish your body right after a tough workout.