TEDx events are independently organized, therefore each team has its own way of selecting speakers. To learn more about speaking at an independent TEDx event, you will need to contact the local organizers of a particular event.
You can reach out to a TEDx team via their public-facing contact information. Basic information about upcoming TEDx events can be found by searching this map: http://www.ted.com/tedx/events
Additionally, most TEDx events have their own websites, Facebook pages, or other social media accounts that contain more information about how to become a speaker at that event. Keep in mind that not all TEDx events accept speaker submissions, and most events select their speakers many months in advance.
Additional information on speaking at a TEDx event can be found here.