A recent question asker asked about girl's favorite festures on a guy. The resounding answer was broad shoulders (among others). So it appears that broad shoulders are a manly feature, however, I don't think that alone would exclude you from being an attractive girl.
my best friend have a broad shoulder and lot of guys think she is sexy and hot. I think what matter the most is how you feel about yourself. Look for the clothes that would cover your broad shoulder or clothes that compliment your look and personality.