[Doug Funnie II]

Don't settle for just mass. If you gain 25 lbs of fat with 5 lbs of muscle you'll just look fat. My first 3 months of serious lifting/diet I gained around 17lbs, probably about 10 of fat and 7 of muscle.

The most important part of weight gain will be diet. You may gain a quick 5 lbs of "muscle" just by starting to lift regularly, but this will probably just be water weight. Most skinny people have little conception of how many calories they eat or what the ratio of protein/fats/carbs they are consuming is. If you keep track of this and make sure to eat enough and often enough, there's no reason that you couldn't gain a very significant amount of muscle.