Hosting a fitness fair (which is what I think you are asking) can be as simple or as complicated as you want it to be. There is a great deal involved in having anything more than a few showcase classes. If you are really looking to do this in a large scale way, I recommend you find a community near you that already hosts fitness events/fairs and attend theirs for ideas. Make contacts there and try to get involved in helping them put their event on. You might then get help from that community to put on your event as well.