Most airline websites have an agent application feature. That’d be a good place to start.
Before you choose to start this business, you might want to take into account the latest plans announced by airlines (and hotel chains) to cut out agencies – or at least slash their commissions – in order to save their SG&A costs. One way they’re planning to do this is by coming out with their own mobile apps which double up as ticketing and loyalty apps and provide enhanced features that are not available to travelers booking tickets via third party agents.
UPDATE DATED 10 JANUARY 2020:
It’s five years since I wrote the above answer. Life has indeed become very tough for travel agents for air tickets, as I’d warned in my original answer written in 2015.
One major airline, JET Airways, is out of business. The future outlook of another major airline, AIR INDIA, is murky. The traditional benefit of booking air tickets via Travel Agents is one-stop shop facility for tickets from multiple airlines. With the number of airlines coming down drastically, this benefit is fast eroding. Many brick-and-mortar travel agents have shut shop. Doubts are being raised on the long term survival of some Online Travel Agencies. In the last year or two, I’ve made most of my air ticket bookings from the website of the respective airline rather than on an OTA website.