In reply to part one--What qualities are needed to be a public speaker?--I have observed the following attributes common to most successful speakers:
Confident
Organized
Outgoing
Engaging
Flexible
Unflappable
Light-hearted
Gracious
I agree with Matthew Manning's reply to part two of the question--How do I speak among a huge crowd?--with only one exception...the use of humor, but there are always exceptions. Here are some of the tips I shared with speakers I've coached over the years:
To begin with, do not for a moment think you cannot give a speech. You give speeches every day...to your family, friends, colleagues, and, yes, even to strangers. Your daily conversations are nothing more than mini-speeches in casual clothes. If you can talk to one person, you can talk to an audience of thousands. Really.