How to run a convention?

Thank you for your question.  
 
However, I believe that to answer it well, your question will require
more time and effort than the average amount of time and effort
associated with this price. Here is a link to guidelines about pricing
your question, in the pricing guide:
https://answers.google.com/answers/pricing.html

It might be helpful to know what kind of convention you have in mind.
A national political convention? A science fiction convention? A sales
convention for some category of products?

The worlds best convention organizers would have to be Jehovahs
Witnesses. They will not do it for you but would help in telling you
the requirements of the participants. Their conventions, based on
Bible teaching [ the teachings of Jesus] and not manmade tradition,
cater for crowds ranging from a few hundred to 150,000 or more. You
could inspect any venue as they departed and not find a single lolley
wrapper on the ground. Just ask the owners of the venues that they
book. No pushing, shoving or foul language and utmost courtesy in
driving away. But if you expect to make a profit from your convention
by inviting the rest of the world, expect trouble.