This may sound funny, but practice :) when you are grocery shopping, greet the clerk. Maybe if you cross paths with someone in an elevator, say hello! Before you practice, however, make sure you are smiling. This may seem akward at first, but it will become more natural. Smiling not only makes you feel better, it also provides an open invitation for someone to strike a conversation. Good luck on your future conversing :)
Conversation is as much listening as it is knowing what to say. Since most people have things they need to vent a sympathetic ear is a big part of conversation. Someone who likes the sound of their own voice too much is always a boor no mater how interesting their words are. Instead of trying to dazzle someone with your verbal skills let them be the center of the conversation.
All good answers so far; I'll just add that it helps if you really like all different kinds of people and enjoy hearing from them. That, too, is something one can learn.
Through practice and being educated so you can have an intelligent conversation. Reading helps to improve your vocabulary.