Accept them for who they are. Introversion is an excellent trait—they typically are very skilled problem-solvers, for instance. Being an extrovert isn't 'better,' so don't try to change your child.
Talk in terms of preferences, rather than defects. For instance, don't say, "She's feeling shy." Instead, you might say, "She likes to know people a little more before she talks about them."
Talk to them one-on-one. Sit down with them each day at a quiet time, like right before bed, and give them a chance to talk. An introverted teenager might not share their thoughts at the dinner table or in a group setting, but these kids are very thoughtful and they have a rich inner life.