Cut down on cardio activities. You may be so active in running, playing basketball, etc, that it's hard to put enough calories in yourself to gain mass. Another thing to keep in mind is that not everyone can get huge, but don't use that as an excuse not to try.
Back to the main topic of this thread: if you're skinny, bulking up will help with girls, but only to a point. My body got the most positive comments from females (of all ages) at the end of high school, when I was 5'11", 190 lbs, and cut. Now, I'm 20-25 lbs heavier, still cut, but I think I've passed the peak musculature for a really sexy body. Women like to see men who are athletic looking, with a good shape to their ass, abs, pecs, shoulders, and calves. They don't seem to be overly impressed when those body parts get huge.
That's not to say being really big doesn't have its advantages: you do get a lot of respect, and it has increased my confidence. If the advantages didn't outweight the disadvantages, I'd cut back down to 190. But if your primary reason for putting on muscle is to get girls, do it in moderation. It's tempting to do too much.