Some men have been complaining that they speak with girls, however, the girls then ignore them, and blow-them-off. This is why I would like to discuss a little bit about why you should be making your conversations relevant to the situation. Don't go off the deep end speaking about a highly specific situation that is very technical in nature. This could be a computer issue, or even an at-length description of somebody's dog. This kind of plays into our other posts, but nevertheless, it can be categorized all-on-its-own. In general, a guy wants to speak generally about things, to see if the girl shows any interest in your conversations. If you have a 'hit', you can digress into a little more detail, and if still then she has interest, go into even deeper detail (if you are knowledgeable on the subject matter), then you will want to release and return to more general info.
So the key, really is to identify who she really is. Make educated guesses, and not just shoot blindly in the dark, asking question after question. This ultimately means you will need to practice your people skills, and learn to pick up on things, that are common amongst all people.
You'll know your not speaking about relevant subject matter if, she excuses herself from your conversation, and leaves. For more information, do a google search about overcoming shyness, or keeping a conversation relevant.