Find something you have mastered, something you are really good at and that is generally interesting. See if there's a place you can offer to teach your skills to someone else. It will be uncomfortable at first, but if you prepare yourself and the material well, you will find that you can speak from a position of authority, and this will build your self confidence. When you discover how to hold people's attention, you will learn a new skill, and you will be able to speak more confidently in other, less comfortable conditions.