TEDx events are independently organized, therefore each team has its own way of selecting volunteers. To learn more about volunteering at an independent TEDx event, you will need to contact the local organizers of the event you wish to help organize.
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 support that event. If an event is looking for volunteers they will often have an application form on their website or will put a call out for new team members on their social media.