One great way to let a girl know you're interested in her is to act like it (genuinely, of course). Ask her questions about her day, her family, her pet hamster, whatever. The point is, you're asking her things that will help you get to know her better. Asking questions is extra helpful when the girl is a bit shy. That way, she can just respond to the conversation without having to lead it, since she might be nervous and unsure of what to say.
Remember, if you're just getting to know each other, don't make the questions too personal. Keep it friendly. The ice will be broken before you know it.