Teenagers, I don't have any idea how to do that with them. But as for anyone older...
I like some of the tips already given. My rule of thumb..... first I try to make sure I'm not seeming too anxious when I approach someone (as much as I am able, anyway), then try to find something mutual to comment on. Usually that's the weather. "Beautiful day out, isn't it?" If I get a good response to that, I keep going. "So what are you up to today?" Hopefully that answer will spawn follow up questions that you can ask about the person. Always try to err on the side of asking about another person's life, rather than talking about your own, when you don't know them very well. If not a good response to initial question, forget it and move on to someone else. This works best with only 1 other person, more than 1 and I can't help you.