When speaking:
Speak clearly and sufficiently loudly-people will be more sympathetic if they can hear and understand easily.
Smile and make eye contact with people to engage them with what you are saying.
Take your time when speaking-aim to talk a little more slowly than usual to avoid rushing if you are nervous.
Be brief: make your point and then stop-avoid going back over what you've said.
Be clear:if something sounds confused you can always say "i'll make that clearer" or check if people have understood.
Don't apologise-it will make you seem less authoritative. Act confident,even if you don't feel it!