It's always the quiet ones you have to worry about. you never know when they'll snap and do someting crazy. You get the courage by simply DOING it, don't think about it. Do it. Geez I sound like a Nike add.

If you're actually looking for an answer... you play out the conversation in your head and try to insert all the possibilities and responses that might come up so you'll feel safer and more prepared when you speak. Then you pray and ask God for an opportunity to speak and you say what's on your mind. It may not go as you would like so you have to be prepared for that. But at least it'll be said and off your mind!