I've seen far too many speakers who make self-denigrating opening remarks. OK, the best person to tell a story against is yourself, but don't do it right at the start of your speech. Worst of all, in my view, is the person who wanders on, looking hunched and nervous, handles the mike as though it's a poisonous snake, and then says "I'm not very good at this, and my speech isn't very well prepared"
We know you're nervous. We know you're not too confident. But we want you to do well. If you appear in front of us and start off by apologising, we'll expect the worst, and that will probably be our perception when you've finished.
So, take a few deep breaths, walk on stage with your head held high and smile. It doesn't matter if you are feeling nervous. We understand. The important thing is to do your best, and the audience will support you. If you lower our expectations by hand-wringing like Uriah Heep (no, the one in David Copperfield, not the elderly rock band), than things will only deteriorate.