Official TED conferences (TED, TEDGlobal, TEDWomen, etc) use TED staff to support the conference, so there are typically no opportunities for volunteers. However, some TEDx events may need the help of volunteers. TEDx events are independently organized, entirely separate from TED, so to inquire if a TEDx event needs help, you can find an event near you and contact the organizing team.
If you can't find contact information on the event page, try searching for the event name online. Most TEDx events have their own websites, Facebook pages, and/or Twitter accounts.
As an alternative way to volunteer, you might consider helping TED transcribe or translate TED talks. You can learn more about how to become a TED Translator in this article.