I wouldn't be overly concerned about gaining weight or adding muscle. Your goals as an athlete should be to improve strength, power, speed, and athletic performance.
Nutrition plays a huge role obviously in gaining weight. So here are a couple of nutritional recommendations if you are serious about adding weight.
Make sure you are getting plenty of protein. Shoot for .75-1.0 grams per pound of body weight. In your case, at least 100 grams daily.
Make sure the extra calories you take in are good, clean calories. Focus on protein and COMPLEX carbs. Sugar is a simple carb and should be eliminated from your as much as possible.
One reason people have trouble gaining weight is that they don't realize how many more calories you need to take in. For your age, you need anywhere between 2000-3000 calories per day depending upon your metabolism and your level of activity. You would need approximately 500 EXTRA calories per day to gain 1 pound a week. But the one pound is probably not going to be lean tissue if you're not training. So, since you want that added weight to be muscle, you start strength training. Well, now your body is expending additional calories during your training. So that extra 500 calories is being used to fuel your workouts. So you have to consume even MORE calories on top of the 500 that you already added. So you can see that if you are doing serious training to gain weight, your calorie intake may be an additional 1000 calories above what you were consuming before you decided to gain weight and start training.
Here's a basic outline on eating for athletes:
http://www.maxxtraining.com/nutrition.htm
