How can I be more confident when I am talking to people?

Unless I am asked a question which I can give a straightforward to, I get extremely shy and anxious. Mainly I think the reason is because I'm worried that if I talk like I naturally do, I will sound unintelligent and be hard to understand. When I'm involved in a conversation that is unpredictable and I haven't had the time to organize my thoughts, I sound very tered and unsure of myself, I use the word "like" a lot, I think of things I wish I had said when it is too late, etc. How can I become more articulate and confident?