This is a bug in our DNA. When we’re on the spot, giving a speech, or improvising an answer at a big meeting, we get nervous.
We sweat and speak too fast. Our voice gets tight. We avoid eye contact and put on a half smile and do many of the things that people often associate with lying. Or, at best, we lack credibility.
So, assuming that you have good intentions, you have 2 options: either avoid speaking in public (bad idea because you need to be visible) or master your nervousness (which comes with practice).
Speak assertively, make great eye contact, talk more slowly.