I think everyone who has posted thus far has the right idea. Practise makes perfect. Expose yourself to situations where you must interact with other people; eventually you'll get comfortable with the idea.
Although, there is one thing I noticed about most introverted types. We tend to focus so much on ourselves when we're interacting with other people that half the time, we're not even full engaged in the conversation. We're constantly worrying about how we come across to others; trying not to say anything stupid; trying not to blush; not to fidget; trying to decipher if the other person is thinking anything negatively about us; fixating on words or gestures, trying to figure out if the other person 'means' anything by it.
Try focusing on the other person for a change. Pay close attention to their facial expressions, the way they're pronouncing their words and modifying their voice; their body language. Focus on making them feel comfortable in your company, and you'll relax a bit more too.