It is not enough to be absorbed by your subject—to acquire self-confidence you must have something in which to be confident. If you go before an audience without any preparation, or previous knowledge of your subject, you ought to be self-conscious—you ought to be ashamed to steal the time of your audience.
Know what you are going to talk about, and, in general, how you are going to say it.
Have the first few sentences worked out completely so that you may not be troubled in the beginning to find words. Know your subject better than your hearers know it, and you have nothing to fear.