Vocational schools, community colleges, continuing education classes, or industry associations offer training. These classes teach students about international traveling regulations, reservation systems, tourism, business, and hospitality. Travel agents can receive accreditation from the Travel Institute, International Airline Transport Association's Training and Development Institute, or the Airlines Reporting Corporation.
If you live in Florida, Hawaii, Nevada, Iowa, Washington, or California, you need to be registered to be able to practice. Those living in Rhode Island need to obtain a license.
Beware of travel agent certification or accreditation mills (like puppy mills, but with less controversy and far less barking). There is no way to instantly become a travel agent, unless you have a very obliging fairy godmother. These fraudulent companies sell people worthless training materials along with a fake license or identification card. One of these card mills was recently bused by the FTC, after which over 70,000 "travel agents" were left without any ability to get their money back. There are 70,000 people who will be booking a trip to Brokesville.
When in doubt of a certificate program, contact The American Society of Travel Agents.