Most speeches begin with someone being given the opportunity to make a speech. I like to use the word opportunity when I talk about speaking. An invitation to speak is an opportunity—not something to be avoided like a root canal or an IRS audit. Let's assume for a moment that you're the one who has been given the opportunity. I can almost hear you saying, "Yes, but I don't have anything worthwhile to talk about."
Nonsense. If somebody didn't think you had something worthwhile to offer, you would never have been invited to speak. Besides, a subject doesn't have to be a world-shaker to be interesting. If the subject is appropriate for the audience, and if the speech is well crafted and well delivered, it will be worthwhile.