I recommend getting it from "the horse's mouth." To do that, visit the Airline Reporting Corp. site (ARC) at www.arccorp.com/join/arc-accredited-travel-agency.jsp. This application will allow you full access to everything that the industry offers as a fully accredited travel agency/agent ($20k min. bond/letter of credit or cash & numerous other requirements will apply). The alternative is to become a VTC (Verified Travel Consultant), where you will be recognized, will have limited access to what ARC offers (therefore less requirements) but you will not be able to issue airline, train or bus tickets (no ticketing privileges). I believe that this is the venue used by what is now commonly known as "Home Based Agents" (I think). The website will clarify that and any other questions you may have.
SECOND OPTION: Quickest/most economic way is for you to become an outside sales representative through association with an already established ARC fully accredited agency (recommended for non-profit organizations where funds are limited). Normally, an agreement is all that is required where a percentage of the commission will revert to you upon receipt from vendors (full payment often required) but you can sell ANYTHING in the industry.
My agency offers that based on: ARC rules us all, payments through credit card in accordance with our requirements, vendor requirements are reservations' primary "rulers", you assume liability for credit memos/disputes related to your sales, you're responsible for cancellations due to missed deadlines, & predetermine the level of your involvement - commission is generally 10% or higher and percentage share is 40/60 (greatest to whoever does most of the work payable upon receipt PLUS you're eligible to get your own IATA or IATAN card based on sales, which gives you personal discounts. We are reputable, establish in the early 40s as a fully accredited agency, so Email me if want to discuss further